avformat/avidec: Fix io_fsize overflow

Fixes: signed integer overflow: 7958120835074169528 * 9 cannot be represented in type 'long long'
Fixes: 23382/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-6230683226996736

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

libavformat/avidec.c

@@ -439,7 +439,7 @@ static int calculate_bitrate(AVFormatContext *s)
         maxpos = FFMAX(maxpos, st->index_entries[j-1].pos);
         lensum += len;
     }
-    if (maxpos < avi->io_fsize*9/10) // index does not cover the whole file
+    if (maxpos < av_rescale(avi->io_fsize, 9, 10)) // index does not cover the whole file
         return 0;
     if (lensum*9/10 > maxpos || lensum < maxpos*9/10) // frame sum and filesize mismatch
         return 0;