dvdsubdec: reset buffer size on invalid over-large packets

Otherwise it will never be reset, and remain "stuck" in this state
forever. Can happen when seeking: the decoder will receive fragments
from different file positions, which triggers the condition easily.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dvdsubdec.c

@@ -519,6 +519,7 @@ static int append_to_cached_buf(AVCodecContext *avctx,
     if (ctx->buf_size >= sizeof(ctx->buf) - buf_size) {
         av_log(avctx, AV_LOG_WARNING, "Attempt to reconstruct "
                "too large SPU packets aborted.\n");
+        ctx->buf_size = 0;
         return AVERROR_INVALIDDATA;
     }
     memcpy(ctx->buf + ctx->buf_size, buf, buf_size);