Keep FFmpeg partial Session teardown null-safe

[Blackspirits]

Summary

Session has two constructor failure paths that call Dispose() before _audioSink has been assigned:

• avformat_find_stream_info failure;
• a successfully opened container with no usable audio/video stream.

The first _audioSink.Reset() null dereference is swallowed by the existing catch, but the later unconditional _audioSink.Dispose() throws. That masks the intended constructor error and prevents the remainder of teardown from disposing the wake events and closing AVFormatContext.

This change:

• makes only the two audio-sink accesses inside Session.Dispose() null-safe;
• leaves normal seek/audio-device behavior unchanged;
• allows partial-construction cleanup to reach avformat_close_input and preserve the original constructor exception.

Validation

• based on upstream c77c4b428649ce7030e6cf218b4d7c384b955232
• one file, two null-safe calls, no public behavior change on a successfully constructed Session
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• UI suite: 5,151 passed, 9 skipped, 0 failed (5,160 total)
• retry step was not used
• validation run: #34779363615

AI assistance: ChatGPT was used to audit FFmpeg constructor failure and teardown ownership paths.

[Blackspirits]

Independent adversarial re-check: Session can call Dispose during constructor failure before _audioSink is assigned. The original Reset null dereference was swallowed, but the later Dispose dereference escaped and prevented AVFormatContext cleanup. The final diff is restricted to null-safe sink access inside Session.Dispose; normal constructed-session behavior is unchanged. Full CI #34779363615 passed restore, build and the complete suite on the first run; UITests reported 5,151 passed, 9 skipped, 0 failed. Retry was not used. No blocker identified. Keep draft; no merge performed.