avformat/utils: Fix integer overflow with duration_gcd in ff_rfps_calculate()

Fixes: signed integer overflow: 136323327 * 281474976710656 cannot be represented in type 'long'
Fixes: 30913/clusterfuzz-testcase-minimized-ffmpeg_dem_IVF_fuzzer-5753392189931520

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

libavformat/utils.c

@@ -3409,7 +3409,8 @@ void ff_rfps_calculate(AVFormatContext *ic)
         // the check for tb_unreliable() is not completely correct, since this is not about handling
         // an unreliable/inexact time base, but a time base that is finer than necessary, as e.g.
         // ipmovie.c produces.
-        if (tb_unreliable(st->internal->avctx) && st->internal->info->duration_count > 15 && st->internal->info->duration_gcd > FFMAX(1, st->time_base.den/(500LL*st->time_base.num)) && !st->r_frame_rate.num)
+        if (tb_unreliable(st->internal->avctx) && st->internal->info->duration_count > 15 && st->internal->info->duration_gcd > FFMAX(1, st->time_base.den/(500LL*st->time_base.num)) && !st->r_frame_rate.num &&
+            st->internal->info->duration_gcd < INT64_MAX / st->time_base.num)
             av_reduce(&st->r_frame_rate.num, &st->r_frame_rate.den, st->time_base.den, st->time_base.num * st->internal->info->duration_gcd, INT_MAX);
         if (st->internal->info->duration_count>1 && !st->r_frame_rate.num
             && tb_unreliable(st->internal->avctx)) {