Unix: fix segfault if ~/.putty/sessions doesn't exist.

Looks as if I introduced this in commit 733fcca2c, where the pointer
returned from enum_settings_start() stopped being the same thing as
the underlying 'DIR *' - I needed to retain a check for the outer
containing structure not being NULL but the DIR * being NULL inside
it.

unix/uxstore.c

@@ -562,6 +562,9 @@ char *enum_settings_next(settings_e *handle, char *buffer, int buflen)
     int maxlen, thislen, len;
     char *unmunged;
 
+    if (!handle->dp)
+        return NULL;
+
     fullpath = make_filename(INDEX_SESSIONDIR, NULL);
     maxlen = len = strlen(fullpath);
 
@@ -592,6 +595,7 @@ char *enum_settings_next(settings_e *handle, char *buffer, int buflen)
 
 void enum_settings_finish(settings_e *handle)
 {
+    if (handle->dp)
         closedir(handle->dp);
     sfree(handle);
 }