avformat/movenc: Don't forget to free fragment buffers

avformat/movenc: Don't forget to free fragment buffers

The buffers used when fragmented output is enabled have up until now not
been freed in the deinit function; they leak e.g. if one errors out of
mov_write_trailer() before one reaches the point where they are normally
written out and freed. This can e.g. happen if allocating new vos_data
fails at the beginning of mov_write_trailer().

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