avformat/utils: reorder duration computation to avoid overflow

Fixes: signed integer overflow: 8 * 9223372036854774783 cannot be represented in type 'long'
Fixes: 23381/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-4818340509122560

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 10cc82c35b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/utils.c

@@ -2775,7 +2775,7 @@ static void estimate_timings_from_bit_rate(AVFormatContext *ic)
                 st      = ic->streams[i];
                 if (   st->time_base.num <= INT64_MAX / ic->bit_rate
                     && st->duration == AV_NOPTS_VALUE) {
-                    duration = av_rescale(8 * filesize, st->time_base.den,
+                    duration = av_rescale(filesize, 8LL * st->time_base.den,
                                           ic->bit_rate *
                                           (int64_t) st->time_base.num);
                     st->duration = duration;