avformat/concatdec: Check user_duration sum

Fixes: 62276/clusterfuzz-testcase-minimized-ffmpeg_dem_CONCAT_fuzzer-6434245599690752
Fixes: signed integer overflow: 9223372026773000000 + 22337000000 cannot be represented in type 'long'

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

@@ -679,6 +679,8 @@ static int concat_read_header(AVFormatContext *avf)
             cat->files[i].user_duration = cat->files[i].outpoint - cat->files[i].inpoint;
         }
         cat->files[i].duration = cat->files[i].user_duration;
+        if (time + (uint64_t)cat->files[i].user_duration > INT64_MAX)
+            return AVERROR_INVALIDDATA;
         time += cat->files[i].user_duration;
     }
     if (i == cat->nb_files) {