Fix hanging issue in _XReply

Assume event queue is empty if another thread is blocking waiting for event.

If one thread was blocking waiting for an event and another thread sent a
reply to the X server, both threads got blocked until an event was
received.

Signed-off-by: Tatu Frisk <tatu.frisk@ge.com>
Signed-off-by: Jose Alarcon <jose.alarcon@ge.com>