avcodec/pcm-dvd: fix incorrectly printed warning about changed block sizes

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

libavcodec/pcm-dvd.c

@@ -248,7 +248,7 @@ static int pcm_dvd_decode_frame(AVCodecContext *avctx, void *data,
 
     if ((retval = pcm_dvd_parse_header(avctx, src)))
         return retval;
-    if (s->last_block_size != s->block_size) {
+    if (s->last_block_size && s->last_block_size != s->block_size) {
         av_log(avctx, AV_LOG_WARNING, "block_size has changed\n");
         s->extra_sample_count = 0;
     }