poll_for_response: Call poll_for_event again if xcb_poll_for_reply fails

If xcb_poll_for_reply fails to find a reply, poll_for_response would
always return NULL. However, xcb_poll_for_reply may have read events
from the display connection while looking for a reply. In that case,
returning NULL from poll_for_response is wrong and can result in the
client hanging, e.g. because it returns to waiting for the display
connection file descriptor becoming readable after XPending incorrectly
returned 0 pending events.

The solution is to call poll_for_event again after xcb_poll_for_reply
returned 0. This will return the first of any events read by
xcb_poll_for_reply.

Fixes issue #79.

Reported-by: Yuxuan Shui <yshuiv7@gmail.com>
Bugzilla: https://bugs.freedesktop.org/108008
Bugzilla: https://bugs.freedesktop.org/107992


Reviewed-by: Adam Jackson <ajax@redhat.com>