Under Wayland, if the mouse pointer is disconnected whilst captured
(like say via KVM switch), the waylandWarpPointer code will be called
but the pointer will be NULL. This results in the cryptic message:
error marshalling arguments for confine_pointer (signature noo?ou): null value passed for arg 2
Error marshalling request: Invalid argument
This patch adds a check on the wlWm.pointer pointer before attempting
to warp the pointer, and avoids the crash.
