wmaprodec: 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/wmaprodec.c

@@ -277,6 +277,11 @@ static av_cold int decode_init(AVCodecContext *avctx)
     int log2_max_num_subframes;
     int num_possible_block_sizes;
 
+    if (!avctx->block_align) {
+        av_log(avctx, AV_LOG_ERROR, "block_align is not set\n");
+        return AVERROR(EINVAL);
+    }
+
     s->avctx = avctx;
     avpriv_float_dsp_init(&s->fdsp, avctx->flags & CODEC_FLAG_BITEXACT);