avformat/movenc: Free old vos_data before overwriting it

Otherwise the old data leaks whenever extradata needs to be rewritten
(e.g. when encoding FLAC with our encoder that sends an updated
extradata packet at the end).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 432f291dff)

libavformat/movenc.c

@@ -6880,6 +6880,7 @@ static int mov_write_trailer(AVFormatContext *s)
             AVCodecParameters *par = track->par;
 
             track->vos_len  = par->extradata_size;
+            av_freep(&track->vos_data);
             track->vos_data = av_malloc(track->vos_len + AV_INPUT_BUFFER_PADDING_SIZE);
             if (!track->vos_data)
                 return AVERROR(ENOMEM);