finalize() located the mfra by reading the trailing mfro with fopen and
fseeko and decoding it by hand, the same job zm_mp4::media_end() does for
the sidx scan. Use media_end() for both, so the last HLS fragment and the
index agree on where the media ends.
media_end() is also stricter: it requires an mfra box of the stated size
at the offset the mfro points to, where the old code accepted any size up
to the file length. A trailer that does not check out now leaves the final
fragment running to EOF, as a missing trailer already did.
A new test covers media_end() against the fixture's real mfra and three
damaged trailers: an mfro size off by four, one larger than the file, and
no mfro at all.
refs #5144
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
