avformat/avidec: Fix memleak with embedded GAB2 subtitles

The code for GAB2 subtitles predates refcounting AVPackets. So in order
to transfer the ownership of a packet's data pkt->data was simply stored
and the packet zeroed; in the end (i.e. in the read_close-function) this
data was then simply freed with av_freep(). This of course leads to a leak
of an AVBufferRef and an AVBuffer. It has been fixed by keeping and
eventually unreferencing the packet's buf instead.

Additionally, the packet is now reset via av_packet_unref().

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
