avformat/utils: Check rfps_duration_sum for overflow

Fixes: signed integer overflow: 9151595917793558550 + 297519050751678697 cannot be represented in type 'long'
Fixes: 15496/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5722866475073536

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

@@ -3348,8 +3348,10 @@ int ff_rfps_add_frame(AVFormatContext *ic, AVStream *st, int64_t ts)
                 }
             }
         }
-        st->info->duration_count++;
-        st->info->rfps_duration_sum += duration;
+        if (st->info->rfps_duration_sum <= INT64_MAX - duration) {
+            st->info->duration_count++;
+            st->info->rfps_duration_sum += duration;
+        }
 
         if (st->info->duration_count % 10 == 0) {
             int n = st->info->duration_count;