mpeg-ts: wrong position after non flushing seeks

Submitted by Vincent Penquerc'h @vincent

Link to original bug (#745102)

Description

Using -base's playback-test program:

./tests/examples/playback/playback-test 0 file://$somempegtsfile

Seeking with the flushing flag off kinda works (though there is image corruption that looks very much like a keyframe is missing), the position will not be updated to the new one after the seek is performed.