fix: handle DTS discontinuity by resetting extractor on error

[keks2293]

fix: recover from DTS extractor error instead of killing the stream

Problem

When DTSExtractor.Extract() returns an error (e.g., non-monotonic DTS on camera
reconnect, stream discontinuity, or DTS > PTS), mediamtx returns the error which
terminates the entire stream.

This happens frequently with IP cameras that lose signal and reconnect, resetting
their DTS to 0. The stream is killed for a temporary condition.

Fix

Instead of propagating the error, reset the DTS extractor to nil and drop the frame.
The extractor will be recreated on the next IDR/random access frame, and the stream
continues normally.

Rationale

• killing the stream on a temporary DTS discontinuity is worse than dropping a single frame
• FFmpeg and GStreamer handle discontinuities the same way: reset timing baseline, continue