avformat/genh: Check block_align

Fixes: infinite loop
Fixes: 26440/clusterfuzz-testcase-minimized-ffmpeg_dem_GENH_fuzzer-5632134020333568

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 37396e9ba8)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/genh.c

@@ -145,6 +145,9 @@ static int genh_read_header(AVFormatContext *s)
         }
     }
 
+    if (st->codecpar->block_align <= 0)
+        return AVERROR_INVALIDDATA;
+
     avio_skip(s->pb, start_offset - avio_tell(s->pb));
 
     avpriv_set_pts_info(st, 64, 1, st->codecpar->sample_rate);