dvdsubdec: implement flushing

This is needed for proper operation with seeking.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dvdsubdec.c

@@ -719,10 +719,15 @@ static av_cold int dvdsub_init(AVCodecContext *avctx)
     return 1;
 }
 
-static av_cold int dvdsub_close(AVCodecContext *avctx)
+static void dvdsub_flush(AVCodecContext *avctx)
 {
     DVDSubContext *ctx = avctx->priv_data;
     ctx->buf_size = 0;
+}
+
+static av_cold int dvdsub_close(AVCodecContext *avctx)
+{
+    dvdsub_flush(avctx);
     return 0;
 }
 
@@ -749,6 +754,7 @@ AVCodec ff_dvdsub_decoder = {
     .priv_data_size = sizeof(DVDSubContext),
     .init           = dvdsub_init,
     .decode         = dvdsub_decode,
+    .flush          = dvdsub_flush,
     .close          = dvdsub_close,
     .priv_class     = &dvdsub_class,
 };