tests: check for NULL surface in keyboard and pointer handlers

When a test destroys a wl_surface, it is still possible to get events
referring to the destroyed surface. The surface in such cases will be
NULL.

Handle NULL surface gracefully in keyboard and pointer enter/leave
handlers. Touch-down handler is already NULL-safe.

This fixes a SEGV in a test I am writing for wp_viewport.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>