Merge pull request #28308 from shahkarnav115-beep:dnn-mvn-defensive-checks

dnn: improve robustness of MVN layer#28308

This change adds small defensive improvements to the MVN layer implementation:

->Guard against zero-sized OpenCL kernel launches

->Add input validation in finalize()

->Use int64 for FLOPS computation to avoid overflow

No functional or performance changes are intended.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/dnn/src/layers/mvn_layer.cpp

@@ -171,7 +171,9 @@ public:
             String buildopt = "-DNUM=4" + opts;
             ocl::Kernel k("mean_fuse4", ocl::dnn::mvn_oclsrc, buildopt + " -DKERNEL_MEAN_FUSE");
             size_t localsize[] = { LOCAL_SIZE };
-            size_t globalsize[] = { (size_t)s[0] / 4 * localsize[0] };
+            size_t groups = std::max<size_t>(1, s[0] / 4);
+            size_t globalsize[] = { groups * localsize[0] };
+
 
             int argId = 0;
             k.set(argId++, ocl::KernelArg::PtrReadOnly(inpMat));
@@ -374,7 +376,7 @@ public:
                            const std::vector<MatShape> &outputs) const CV_OVERRIDE
     {
         CV_UNUSED(outputs); // suppress unused variable warning
-        long flops = 0;
+        int64 flops = 0;
         for(int i = 0; i < inputs.size(); i++)
         {
             flops += 6*total(inputs[i]) + 3*total(inputs[i], 0, normVariance ? 2 : 1);