avcodec/dvdsubdec: free subtitle rectangles if nothing is output

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

libavcodec/dvdsubdec.c

@@ -540,6 +540,7 @@ static int dvdsub_decode(AVCodecContext *avctx,
 
     if (is_menu < 0) {
     no_subtitle:
+        reset_rects(sub);
         *data_size = 0;
 
         return buf_size;