Destroying surface with active touchpoints leads to crashes

Reproduced with sway, phoc and rootston.

Steps to reproduce:

1. Launch any app, say, weston-terminal
2. Touch it and don't release.
3. Kill weston-terminal (for instance via ssh)
4. Move or release the finger.

Result: events are being sent on a surface that has been already destroyed, leading to a crash.

https://github.com/swaywm/wlroots/issues/1788