muxer: fix nil deref when finalizing muxerSegmentFMP4 with no currentPart

[database64128]

Playing an AV1 + AAC HLS stream published by OBS Studio via RTMP results in the following panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x60 pc=0x560447ec0e43]
goroutine 663 [running]:
github.com/bluenviron/gohlslib.(*muxerSegmentFMP4).finalize(0xc0006101c0, 0x0)
        github.com/bluenviron/gohlslib@v1.2.1/muxer_segment_fmp4.go:83 +0x23
github.com/bluenviron/gohlslib.(*muxerSegmenterFMP4).close(0xc00016a240)
        github.com/bluenviron/gohlslib@v1.2.1/muxer_segmenter_fmp4.go:131 +0x29
github.com/bluenviron/gohlslib.(*Muxer).Close(0xc000404140)
        github.com/bluenviron/gohlslib@v1.2.1/muxer.go:202 +0x38
github.com/bluenviron/mediamtx/internal/servers/hls.(*muxer).runInner(0xc0002a0120, {0x560448714258, 0xc0003da500}, 0xc0011f3fa0?)
        github.com/bluenviron/mediamtx/internal/servers/hls/muxer.go:288 +0xb30
github.com/bluenviron/mediamtx/internal/servers/hls.(*muxer).run.func1.1.1()
        github.com/bluenviron/mediamtx/internal/servers/hls/muxer.go:140 +0x32
created by github.com/bluenviron/mediamtx/internal/servers/hls.(*muxer).run.func1.1 in goroutine 662
        github.com/bluenviron/mediamtx/internal/servers/hls/muxer.go:139 +0x170

This commit addresses the panic, but does not fix the underlying issue with this particular codec combination.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (7413f03) 72.05% compared to head (c614797) 72.00%.

Files	Patch %	Lines
muxer_segment_fmp4.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
- Coverage   72.05%   72.00%   -0.06%     
==========================================
  Files          43       43              
  Lines        3822     3825       +3     
==========================================
  Hits         2754     2754              
- Misses        852      854       +2     
- Partials      216      217       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[aler9]

Hello, thanks for posting the crash dump. I was able to replicate the issue, and in my case it happened when hlsDirectory is not empty and it points to an invalid directory. A more complete fix was added with #137. Please let me know if it solves the crash.

[database64128]

Thanks. I can confirm that #137 does fix the issue for me.