avformat/avidec: Use 64bit in get_duration()

Fixes: signed integer overflow: 2147483424 + 8224 cannot be represented in type 'int'
Fixes: 29619/clusterfuzz-testcase-minimized-ffmpeg_dem_AVI_fuzzer-5191424373030912

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 a0ceb0cdd4)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/avidec.c

@@ -125,7 +125,7 @@ static inline int get_duration(AVIStream *ast, int len)
     if (ast->sample_size)
         return len;
     else if (ast->dshow_block_align)
-        return (len + ast->dshow_block_align - 1) / ast->dshow_block_align;
+        return (len + (int64_t)ast->dshow_block_align - 1) / ast->dshow_block_align;
     else
         return 1;
 }