This diff contains a patch that fixes a crash that I found when using mpv. The crash occurs with following clauses and steps:

1. Use AMD GPU.
2. Use mpv with gpu-next video output (probably current gpu output is affected too, haven't tried it) and VAAPI hwdec.
3. Watch a high-bitrate video or livestream and try to seek to the timestamp that wasn't cached yet.

You'll likely get a segfault which traces into radeonsi_drv_video.so. This patch adds the necessary nullptr check to prevent this segfault.

Watched a video that I wasn't able to reliably seek previosly when using VAAPI. Now I can seek to any timestamp, and while I can get a few dropped frames, it doesn't crash anymore.