libmp4: rewind stream before falling back to avformat

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mp4/libmp4.c

@@ -3609,6 +3609,7 @@ MP4_Box_t *MP4_BoxGetRoot( stream_t *s )
 
 error:
     free( p_root );
+    stream_Seek( p_stream, 0 );
     return NULL;
 }