avformat/flacenc: Fix memleak when writing attached pictures fails
The FLAC muxer currently stores an attached picture corresponding to an
AVStream in AVStream.priv_data. The AVPacket contained therein is
unreferenced after it has been written. The AVPacket structure itself is
then freed generically as AVStream.priv_data.
And this can lead to memleaks if an attached picture is not written:
It might be because the trailer is never written or because writing
a previous attached picture failed in case error_recognition is set
to explode.
Therefore free the packets properly (i.e. with av_packet_free())
in the muxer's deinit function.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>