optflow: fix RLOF SR_CROSS check

Author: alalek

modules/optflow/src/rlofflow.cpp

@@ -281,18 +281,23 @@ class SparseRLOFOpticalFlowImpl : public SparseRLOFOpticalFlow
         CV_Assert(!prevImg.empty() && prevImg.depth() == CV_8U && (prevImg.channels() == 3 || prevImg.channels() == 1));
         CV_Assert(!nextImg.empty() && nextImg.depth() == CV_8U && (nextImg.channels() == 3 || nextImg.channels() == 1));
         CV_Assert(prevImg.sameSize(nextImg));
-        if ((param->supportRegionType == SR_CROSS) && (prevImg.channels() != 3 || nextImg.channels() != 3))
-            CV_Error(cv::Error::BadNumChannels, "if SR_CROSS is used, both images need to have 3 channels.");
-
-        Mat prevImage = prevImg.getMat();
-        Mat nextImage = nextImg.getMat();
-        Mat prevPtsMat = prevPts.getMat();
 
         if (param.empty())
         {
-            param = Ptr<RLOFOpticalFlowParameter>(new RLOFOpticalFlowParameter);
+            param = makePtr<RLOFOpticalFlowParameter>();
+        }
+        CV_DbgAssert(!param.empty());
+
+        if (param->supportRegionType == SR_CROSS)
+        {
+            CV_CheckChannelsEQ(prevImg.channels(), 3, "SR_CROSS mode requires images with 3 channels");
+            CV_CheckChannelsEQ(nextImg.channels(), 3, "SR_CROSS mode requires images with 3 channels");
         }
 
+        Mat prevImage = prevImg.getMat();
+        Mat nextImage = nextImg.getMat();
+        Mat prevPtsMat = prevPts.getMat();
+
         if (param->useInitialFlow == false)
             nextPts.create(prevPtsMat.size(), prevPtsMat.type(), -1, true);