avcodec/avcodec, avpacket: Return blank packet on av_packet_ref() failure

Up until now, it was completely unspecified what the content of the
destination packet dst was on error. Depending upon where the error
happened calling av_packet_unref() on dst might be dangerous.

This commit changes this by making sure that dst is blank on error, so
unreferencing it again is safe (and still pointless). This behaviour is
documented.

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