avutil/mathematics: Use av_sat_add64() for the last addition in av_add_stable()

Fixes: signed integer overflow: 9223372036854770375 + 5450 cannot be represented in type 'long'
Fixes: 26471/clusterfuzz-testcase-minimized-ffmpeg_dem_MXG_fuzzer-6229617557635072

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

libavutil/mathematics.c

@@ -210,6 +210,6 @@ int64_t av_add_stable(AVRational ts_tb, int64_t ts, AVRational inc_tb, int64_t i
         if (old == INT64_MAX || old == AV_NOPTS_VALUE || old_ts == AV_NOPTS_VALUE)
             return ts;
 
-        return av_rescale_q(old + 1, inc_tb, ts_tb) + (ts - old_ts);
+        return av_sat_add64(av_rescale_q(old + 1, inc_tb, ts_tb), ts - old_ts);
     }
 }