avcodec/utils: Check block_align

Fixes: out of array access
Fixes: 18432/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMAV2_fuzzer-5675574936207360
Fixes: 18326/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMAV2_fuzzer-5071752362721280
Fixes: 18384/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMAV1_fuzzer-5769439500304384

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

libavcodec/utils.c

@@ -694,6 +694,11 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *code
         ret = AVERROR(EINVAL);
         goto free_and_end;
     }
+    if (avctx->block_align < 0) {
+        av_log(avctx, AV_LOG_ERROR, "Invalid block align: %d\n", avctx->block_align);
+        ret = AVERROR(EINVAL);
+        goto free_and_end;
+    }
 
     avctx->codec = codec;
     if ((avctx->codec_type == AVMEDIA_TYPE_UNKNOWN || avctx->codec_type == codec->type) &&