mov: zero sc->pb in mov_read_close()

Its bad to free things without zeroing them.
This fixes a potential issue when mov_read_close() would be called twice.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -2782,6 +2782,7 @@ static int mov_read_close(AVFormatContext *s)
         av_freep(&sc->drefs);
         if (sc->pb && sc->pb != s->pb)
             avio_close(sc->pb);
+        sc->pb = NULL;
         av_freep(&sc->chunk_offsets);
         av_freep(&sc->keyframes);
         av_freep(&sc->sample_sizes);