If the length of the `ExtractorInput` is not known then the
`subtitleData` field is re-sized by 1kB each time
(`SubtitleExtractor.DEFAULT_BUFFER_SIZE`), so the end of the array is
often not populated. This change ensures that `length` is propagated to
`SubtitleParser`, so that implementations don't try and parse the
garbage/zero bytes at the end of the array.
Discovered while investigating Issue: androidx/media#1516
#cherrypick
PiperOrigin-RevId: 661195634
f37f9690f4