The padding data is assumed to be 0 in several places, notably in
subtitles. This problem was not detected with fate-sub-srt test because
the first element of the side data (x1) is 0 in the test, so the
trailing side data present in the packet wasn't read by the decoder. The
issue can be observed with a large enough x1.

It is also noted in FF_INPUT_BUFFER_PADDING_SIZE doxy that MPEG
bitstreams require that padding with 0, so it might fix other issues.