decoder: fix pf_decode_* and pf_packetize behavior
decoder: fix pf_decode_* and pf_packetize behavior

Audio, video, and sub decoders were not drained. Indeed, pf_decode_* functions
were called with *pp_block == NULL instead of pp_block == NULL. There was no
way to distinguish a drain from an other call from the decoder loop.

There was also an issue with the packetizer. Indeed, a successful call to
pf_packetize leaded to a drain.

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc