libhb: Plug some leaks and clean up the buffer pool code a little.
Filters were leaking buffers when a job is cancelled.
decavcodec could leak when job cancelled.
decavcodec leaked audio extradata in BSInfo
encavcodec and encavcodecaudio leaked AVCodecContext
sync leaked subtitle_sanitizer_t data


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5853 b64f7644-9d1e-0410-96f1-a4d463321fa5