mpegtsdemux: Fix multiple-SI detection/parsing

mpegtsdemux: Fix multiple-SI detection/parsing

We had two issues with the previous code:
1) We were badly handling PUSI-flagged packets. We were discarding the
   initial data (if pointer != 0) whereas we should have been accumulating
   it with the previous data (if there was a continuity of course).
   => First series of information loss
2) We were not checking whether there were more sections after the end
   of one (i.e. when the following byte was not a stuff byte).

This fixes those two issues.

Fixes #677443

https://bugzilla.gnome.org/show_bug.cgi?id=677443