url: restore the functionality of 'curl -u :'

This commit fixes a regression introduced in
fddb7b44a7.

Reported by: Markus Moeller
Bug: http://curl.haxx.se/mail/archive-2013-06/0052.html

RELEASE-NOTES

@@ -28,6 +28,7 @@ This release includes the following bugfixes:
  o c-ares: improve error message on failed resolve [4]
  o printf: make sure %x are treated unsigned
  o formpost: better random boundaries [5]
+ o url: restore the functionality of 'curl -u :' [6]
 
 This release includes the following known bugs:
 
@@ -48,3 +49,4 @@ References to bug reports and discussions on issues:
  [3] = http://curl.haxx.se/bug/view.cgi?id=1249
  [4] = http://curl.haxx.se/bug/view.cgi?id=1191
  [5] = http://curl.haxx.se/bug/view.cgi?id=1251
+ [6] = http://curl.haxx.se/mail/archive-2013-06/0052.html

lib/url.c

@@ -318,6 +318,13 @@ static CURLcode setstropt_userpwd(char *option, char **userp, char **passwdp,
   if(!result) {
     /* Store the username part of option if required */
     if(userp) {
+      if(!user && option && option[0] == ':') {
+        /* Allocate an empty string instead of returning NULL as user name */
+        user = strdup("");
+        if(!user)
+          result = CURLE_OUT_OF_MEMORY;
+      }
+
       Curl_safefree(*userp);
       *userp = user;
     }