Use correct variable in NULL check

If you pass NULL as the fourth argument to notify_get_server_info it
segfaults.

https://bugzilla.gnome.org/show_bug.cgi?id=623096
(cherry picked from commit 4830863bab)

libnotify/notify.c

@@ -340,7 +340,7 @@ notify_get_server_info (char **ret_name,
                 g_free (version);
         }
 
-        if (spec_version != NULL) {
+        if (ret_spec_version != NULL) {
                 *ret_spec_version = spec_version;
         } else {
                 g_free (spec_version);