dca: Error out on missing DSYNC

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/dcadec.c

@@ -1256,6 +1256,7 @@ static int dca_subsubframe(DCAContext *s, int base_channel, int block_index)
 #endif
         } else {
             av_log(s->avctx, AV_LOG_ERROR, "Didn't get subframe DSYNC\n");
+            return AVERROR_INVALIDDATA;
         }
     }