gfileutils: Add missing g_free() in error path

Discovered by static analysis.

https://bugzilla.gnome.org/show_bug.cgi?id=733576

glib/gfileutils.c

@@ -2013,7 +2013,7 @@ g_file_read_link (const gchar  *filename,
           set_file_error (error,
                           filename,
                           _("Failed to read the symbolic link '%s': %s"));
-	
+          g_free (buffer);
           return NULL;
         }