avformat/concatdec: clip outpoint - inpoint overflow in get_best_effort_duration()

An alternative would be to limit all time/duration fields to below 64bit

Fixes: signed integer overflow: -93000000 - 9223372036839000000 cannot be represented in type 'long long'
Fixes: 64546/clusterfuzz-testcase-minimized-ffmpeg_dem_CONCAT_fuzzer-5110813828186112

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

libavformat/concatdec.c

@@ -323,7 +323,7 @@ static int64_t get_best_effort_duration(ConcatFile *file, AVFormatContext *avf)
     if (file->user_duration != AV_NOPTS_VALUE)
         return file->user_duration;
     if (file->outpoint != AV_NOPTS_VALUE)
-        return file->outpoint - file->file_inpoint;
+        return av_sat_sub64(file->outpoint, file->file_inpoint);
     if (avf->duration > 0)
         return avf->duration - (file->file_inpoint - file->file_start_time);
     if (file->next_dts != AV_NOPTS_VALUE)