When ExtractorInput.getLength() returns a defined length, the
SubtitleExtractor will create a buffer of the same length, call
ExtractorInput.read() until it has read the input bytes, plus one more
time where ExtractorInput.read() returns RESULT_END_OF_INPUT. The last
call to ExtractorInput.read() however will make the SubtitleExtractor to
increase its buffer (including a copy) unnecessarily.
This change makes the SubtitleExtractor avoid calling
ExtractorInput.read() if the expected number of bytes have already
been read, so that the internal buffer does not grow.
PiperOrigin-RevId: 402583610
