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.
Merge pull request #13 from wjt/fix-double-X_UNLOCK-on-xrandr-event

x11vnc: fix double X_UNLOCK on xrandr events