docs: mention NULL is fine input to several functions

Fixes #2837
Closes #2858
Reported-by: Markus Elfring

docs/libcurl/curl_easy_cleanup.3

@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * Copyright (C) 1998 - 2015, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
@@ -47,6 +47,9 @@ Any use of the \fBhandle\fP after this function has been called and have
 returned, is illegal. \fIcurl_easy_cleanup(3)\fP kills the handle and all
 memory associated with it!
 
+Passing in a NULL pointer in \fIhandle\fP will make this function return
+immediately with no action.
+.SH "OLD TIMES"
 For libcurl versions before 7.17,: after you've called this function, you can
 safely remove all the strings you've previously told libcurl to use, as it
 won't use them anymore now.

docs/libcurl/curl_formfree.3

@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
@@ -40,6 +40,9 @@ the \fIcurl_formadd(3)\fP invoke(s).
 
 \fBform\fP is the pointer as returned from a previous call to
 \fIcurl_formadd(3)\fP and may be NULL.
+
+Passing in a NULL pointer in \fIform\fP will make this function return
+immediately with no action.
 .SH AVAILABILITY
 Deprecated in 7.56.0.
 .SH RETURN VALUE

docs/libcurl/curl_free.3

@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * Copyright (C) 1998 - 2014, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
@@ -31,5 +31,8 @@ curl_free - reclaim memory that has been obtained through a libcurl call
 curl_free reclaims memory that has been obtained through a libcurl call.  Use
 \fIcurl_free(3)\fP instead of free() to avoid anomalies that can result from
 differences in memory management between your application and libcurl.
+
+Passing in a NULL pointer in \fIptr\fP will make this function return
+immediately with no action.
 .SH "SEE ALSO"
 .BR curl_easy_unescape "(3), " curl_easy_escape "(3) "

docs/libcurl/curl_mime_free.3

@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
@@ -40,6 +40,8 @@ not be explicitly freed as they are by the top structure freeing.
 \fBmime\fP is the handle as returned from a previous call to
 \fIcurl_mime_init(3)\fP and may be NULL.
 
+Passing in a NULL pointer in \fImime\fP will make this function return
+immediately with no action.
 .SH AVAILABILITY
 As long as at least one of HTTP, SMTP or IMAP is enabled. Added in 7.56.0.
 .SH RETURN VALUE

docs/libcurl/curl_multi_cleanup.3

@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
@@ -40,6 +40,9 @@ handle is no longer connected to the multi handle
 
 3 - \fIcurl_multi_cleanup(3)\fP should be called when all easy handles are
 removed
+
+Passing in a NULL pointer in \fImulti_handle\fP will make this function return
+CURLM_BAD_HANDLE immediately with no other action.
 .SH RETURN VALUE
 CURLMcode type, general libcurl multi interface error code. On success,
 CURLM_OK is returned.

docs/libcurl/curl_share_cleanup.3

@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
@@ -31,6 +31,8 @@ curl_share_cleanup - Clean up a shared object
 This function deletes a shared object. The share handle cannot be used anymore
 when this function has been called.
 
+Passing in a NULL pointer in \fIshare_handle\fP will make this function return
+immediately with no action.
 .SH RETURN VALUE
 CURLSHE_OK (zero) means that the option was set properly, non-zero means an
 error occurred as \fI<curl/curl.h>\fP defines. See the \fIlibcurl-errors.3\fP

docs/libcurl/curl_slist_free_all.3

@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
@@ -30,6 +30,9 @@ curl_slist_free_all - free an entire curl_slist list
 .SH DESCRIPTION
 curl_slist_free_all() removes all traces of a previously built curl_slist
 linked list.
+
+Passing in a NULL pointer in \fIlist\fP will make this function return
+immediately with no action.
 .SH RETURN VALUE
 Nothing.
 .SH EXAMPLE