avformat/mpegts: Don't leave context in inconsistent state upon error

avformat/mpegts: Don't leave context in inconsistent state upon error

Up until now, opening a section filter works as follows: A filter is
opened and (on success) attached to the MpegTSContext. Then a buffer for
said filter is allocated and upon success attached to the section
filter; on error, the filter is simply freed without removing it from
the MpegTSContext, leaving the latter in an inconsistent state. This
leads to use-after-frees lateron.

This commit fixes this by allocating the buffer first; the filter is
only opened if the buffer could be successfully allocated.

Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>