Set errno to 0 before calling strtoul

We check errno for failure, so ensure its cleared first. (#573922)

gtk/gtkwindow.c

@@ -396,6 +396,7 @@ extract_time_from_startup_id (const gchar* startup_id)
       /* Skip past the "_TIME" part */
       timestr += 5;
 
+      errno = 0;
       timestamp = strtoul (timestr, &end, 0);
       if (end != timestr && errno == 0)
         retval = timestamp;