avformat/mux: Fix leaks on error when writing noninterleaved uncoded frames

avformat/mux: Fix leaks on error when writing noninterleaved uncoded frames

If writing uncoded frames in noninterleaved mode fails at the preparatory
steps (i.e. before it reaches write_packet()), the packet would not be
unreferenced and the frame would leak. This is fixed by unreferencing
the packet in write_uncoded_frame_internal() instead.

This also makes it possible to remove the unreferencing in
write_packet() itself: In noninterleaved mode frames are now freed in
write_uncoded_frame_internal(), while they are freed in interleaved
mode when their containing packet gets unreferenced (like normal
packets).

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