avdevice/xcbgrab: check return values of xcb query functions

avdevice/xcbgrab: check return values of xcb query functions

Fixes #7312, segmentation fault on close of X11 server

xcb_query_pointer_reply() and xcb_get_geometry_reply() can return NULL
if e.g. the X server closes or the connection is lost. This needs to
be checked in order to cleanly exit, because the returned pointers are
dereferenced later.

Signed-off-by: Moritz Barsnick <barsnick@gmx.net>
Reviewed-by: Andriy Gelman <andriy.gelman@gmail.com>