avcodec/dvdsubdec: discard accumulated buffer on error

Fixes: Timeout
Fixes: 10992/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DVDSUB_fuzzer-5657495410835456

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dvdsubdec.c

@@ -595,6 +595,7 @@ static int dvdsub_decode(AVCodecContext *avctx,
     }
 
     if (is_menu < 0) {
+        ctx->buf_size = 0;
     no_subtitle:
         reset_rects(sub);
         *data_size = 0;