x11grab: Check XFixesGetCursorImage return value

It could return NULL if the cursor is outside the screen, the connection
timed out or the system is out of memory.

CC: libav-stable@libav.org

libavdevice/x11grab.c

@@ -392,6 +392,8 @@ static void paint_mouse_pointer(XImage *image, X11GrabContext *s)
         return;
 
     xcim = XFixesGetCursorImage(dpy);
+    if (!xcim)
+        return;
 
     x = xcim->x - xcim->xhot;
     y = xcim->y - xcim->yhot;