avconv: make sure packets put into the muxing FIFO are refcounted

avconv: make sure packets put into the muxing FIFO are refcounted

Some callers (like do_subtitle_out(), or do_streamcopy()) call this
with an AVPacket that is not refcounted. This can cause undefined
behavior.

Calling av_packet_move_ref() does not make a packet refcounted if it
isn't yet. (And it can't be made to, because it always succeeds,
and can't return ENOMEM.)

Call av_packet_ref() instead to make sure it's refcounted.

Cc: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>