avcodec/wmalosslessdec: Check block_align

Fixes: NULL pointer dereference
Fixes: 18331/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-5652847445671936

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wmalosslessdec.c

@@ -184,8 +184,8 @@ static av_cold int decode_init(AVCodecContext *avctx)
     unsigned int channel_mask;
     int i, log2_max_num_subframes;
 
-    if (!avctx->block_align) {
-        av_log(avctx, AV_LOG_ERROR, "block_align is not set\n");
+    if (avctx->block_align <= 0) {
+        av_log(avctx, AV_LOG_ERROR, "block_align is not set or invalid\n");
         return AVERROR(EINVAL);
     }