avformat/dsfdec: Check block_align more completely

Fixes: infinite loop
Fixes: 26865/clusterfuzz-testcase-minimized-ffmpeg_dem_DSF_fuzzer-5649473830912000

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

libavformat/dsfdec.c

@@ -124,8 +124,8 @@ static int dsf_read_header(AVFormatContext *s)
 
     dsf->audio_size = avio_rl64(pb) / 8 * st->codecpar->channels;
     st->codecpar->block_align = avio_rl32(pb);
-    if (st->codecpar->block_align > INT_MAX / st->codecpar->channels) {
-        avpriv_request_sample(s, "block_align overflow");
+    if (st->codecpar->block_align > INT_MAX / st->codecpar->channels || st->codecpar->block_align <= 0) {
+        avpriv_request_sample(s, "block_align invalid");
         return AVERROR_INVALIDDATA;
     }
     st->codecpar->block_align *= st->codecpar->channels;