avformat/utils: free existing extradata before trying to allocate a new one

This prevents leaks in the rare cases the function is called when extradata
already exists.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/utils.c

@@ -3245,6 +3245,7 @@ int ff_alloc_extradata(AVCodecParameters *par, int size)
 {
     int ret;
 
+    av_freep(&par->extradata);
     if (size < 0 || size >= INT32_MAX - AV_INPUT_BUFFER_PADDING_SIZE) {
         par->extradata = NULL;
         par->extradata_size = 0;