fix a bug in MergeDebevec (modify input times)

modules/photo/src/merge.cpp

@@ -85,7 +85,7 @@ public:
         CV_Assert(log_response.rows == LDR_SIZE && log_response.cols == 1 &&
                   log_response.channels() == channels);
 
-        Mat exp_values(times);
+        Mat exp_values(times.clone());
         log(exp_values, exp_values);
 
         result = Mat::zeros(size, CV_32FCC);