avformat/flvdec: Use av_freep() avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 91ea466551)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/flvdec.c

@@ -623,7 +623,7 @@ static int flv_read_close(AVFormatContext *s)
 
 static int flv_get_extradata(AVFormatContext *s, AVStream *st, int size)
 {
-    av_free(st->codec->extradata);
+    av_freep(&st->codec->extradata);
     if (ff_get_extradata(st->codec, s->pb, size) < 0)
         return AVERROR(ENOMEM);
     return 0;