backends/x11: Prevent segfault when querying list of devices

The XIQueryDevice function used by device_query_area can return a NULL
pointer and set n_devices to a negative number in some cases. We add
additional checks to prevent a segfault.

https://bugzilla.gnome.org/show_bug.cgi?id=787649

src/backends/x11/meta-input-settings-x11.c

@@ -589,7 +589,7 @@ device_query_area (ClutterInputDevice *device,
   *width = *height = 0;
   device_id = clutter_input_device_get_device_id (device);
   info = XIQueryDevice (xdisplay, device_id, &n_devices);
-  if (n_devices == 0)
+  if (n_devices <= 0 || !info)
     return FALSE;
 
   abs_x = XInternAtom (xdisplay, "Abs X", True);