x11grab: Check the XQueryPointer return value

The paint_mouse_pointer() code uses XFixes to retrieve the cursor
coordinates, but XFixes gives no information about which screen the
pointer is on; this results in always drawing the cursor on the
captured screen even if the mouse pointer was on another screen.

For example, when capturing from screen 1 (i.e. -f x11grab -i ":0.1")
the cursor was being drawn in the captured image even when the mouse
pointer was actually on screen 0, which is wrong and visually confusing.

CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
