wmadec: require block_align to be set.

Avoids an infinite loop in the calling programs with decoder not
consuming any input and not returning output.

CC:libav-stable@libav.org

libavcodec/wmadec.c

@@ -72,6 +72,11 @@ static int wma_decode_init(AVCodecContext * avctx)
     int i, flags2;
     uint8_t *extradata;
 
+    if (!avctx->block_align) {
+        av_log(avctx, AV_LOG_ERROR, "block_align is not set\n");
+        return AVERROR(EINVAL);
+    }
+
     s->avctx = avctx;
 
     /* extract flag infos */