avformat/utils: free AVStream.codec properly in free_stream()

Fixes memory leaks.
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit b9d2005e)

libavformat/utils.c

@@ -4146,9 +4146,7 @@ static void free_stream(AVStream **pst)
     av_freep(&st->index_entries);
 #if FF_API_LAVF_AVCTX
 FF_DISABLE_DEPRECATION_WARNINGS
-    av_freep(&st->codec->extradata);
-    av_freep(&st->codec->subtitle_header);
-    av_freep(&st->codec);
+    avcodec_free_context(&st->codec);
 FF_ENABLE_DEPRECATION_WARNINGS
 #endif
     av_freep(&st->priv_data);