When an error happened in ff_interleave_add_packet() when adding
a packet to the packet queue, said packet would not be unreferenced
in ff_interleave_add_packet(), but would be zeroed in
av_interleaved_write_frame(), which results in a memleak.

This has been fixed: ff_interleave_add_packet() now always unreferences
the input packet on error; as a result, it always returns blank packets
which has been documented. Relying on this a call to av_packet_unref()
in ff_audio_rechunk_interleave() can be removed.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>