scrcpy ignores signals received concurrently with X server dying

- [x] I have read the [FAQ](https://github.com/Genymobile/scrcpy/blob/master/FAQ.md).
- [x] I have searched in existing [issues](https://github.com/Genymobile/scrcpy/issues).

**Environment**
- OS: Gentoo
- scrcpy version: 1.24
- installation method: ebuild
- device model: Pixel XL
- Android version: 11

**Describe the bug**

If I kill scrcpy and the X server at the same time, scrcpy fails to exit, and instead floods the terminal with:

```
WARN: Could not post new frame event:
```

No source file or test is named. Reproduce the failure by stopping scrcpy and the X server concurrently, then trace the signal and X-server shutdown handling; done means scrcpy exits cleanly without flooding the terminal with repeated frame-event warnings.

Written by the indexing model from the issue text.