Fix integer overflow when accumulating timing values.

modules/dnn/src/dnn.cpp

@@ -3074,7 +3074,7 @@ void Net::setHalideScheduler(const String& scheduler)
 int64 Net::getPerfProfile(std::vector<double>& timings)
 {
     timings = std::vector<double>(impl->layersTimings.begin() + 1, impl->layersTimings.end());
-    int64 total = std::accumulate(timings.begin(), timings.end(), 0);
+    int64 total = (int64)std::accumulate(timings.begin(), timings.end(), 0.0);
     return total;
 }