mpegtspacketizer: memcmp potentially seen_before data

Theoretically the version number is incremented every time there's a new section, but in a world of streaming we can't easily make that assumption.

An example of a broken use case is when we're cat-ing two mpeg-ts files together, which is equivalent of capturing a DVB stream while switching channels. A set-top box would know that we switched the channels and reset the demuxer, but in practice this might not happen.