lavf: Make sure avg_frame_rate can be calculated without integer overflow

If either of the deltas is too large for the multiplications to
succeed, don't use this for setting the avg frame rate.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Cc: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/utils.c

@@ -2355,6 +2355,9 @@ int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
                 int      best_fps = 0;
                 double best_error = 0.01;
 
+                if (delta_dts     >= INT64_MAX / st->time_base.num ||
+                    delta_packets >= INT64_MAX / st->time_base.den)
+                    continue;
                 av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
                           delta_packets*(int64_t)st->time_base.den,
                           delta_dts*(int64_t)st->time_base.num, 60000);