x11vnc: fix double X_UNLOCK on xrandr events

check_xrandr_event() assumes X_LOCK is taken before it is called, and
currently calls X_UNLOCK on behalf of the caller. But in practice, all
callers assume that the lock is still held after check_xrandr_event()
returns. In particular, this leads to a double-unlock and crash in
check_xevents() on any xrandr event.

x11vnc/xrandr.c

@@ -268,6 +268,7 @@ int check_xrandr_event(char *msg) {
 			/* under do_change caller normally returns before its X_UNLOCK */
 			X_UNLOCK;
 			handle_xrandr_change(rev->width, rev->height);
+			X_LOCK;
 		}
 		if (qout) {
 			return do_change;