Validate surface is valid when sending TextInput leave event

Summary:
It's possible for the surface to be unbound when we send the leave
event; we've called Resource::unbind() of Surface, so the Surface has,
deleteLater called, but it's still a valid object, and the first check
passes.

We get in this situation because when a surface is destroyed, we're
handling text input from the same source event.

Sending a nullpointer is a protocol error, and wayland kindly closes the
connection.

This fixes my constant:
"Did the Wayland server die" error messages when running clients.

Test Plan:
Got errors after setting up qt virtual keyboard.
Had reproducible case.
Restarted kwin after this patch, now doesn't crash.

Reviewers: #plasma, graesslin

Subscribers: apol, graesslin, plasma-devel, #frameworks

Tags: #plasma_on_wayland, #frameworks

Differential Revision: https://phabricator.kde.org/D5712