Merge pull request #26996 from shyama7004:outofBound

Fix Logical defect in FilterSpecklesImpl #26996

Fixes : #24963

### 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
- [x] 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/calib3d/src/stereosgbm.cpp

@@ -2345,21 +2345,33 @@ void filterSpecklesImpl(cv::Mat& img, int newVal, int maxSpeckleSize, int maxDif
     using namespace cv;
 
     int width = img.cols, height = img.rows, npixels = width*height;
-    size_t bufSize = npixels*(int)(sizeof(Point2s) + sizeof(int) + sizeof(uchar));
+    // space allocation for:
+    // labels : npixels * sizeof(int)
+    // wbuf   : npixels * sizeof(Point2s)
+    // rtype  : (npixels + 1) * sizeof(uchar)
+    size_t bufSize = npixels * sizeof(int)            // for labels
+                     + npixels * sizeof(Point2s)      // for wavefront buffer (wbuf)
+                     + (npixels + 1) * sizeof(uchar); // for region type (rtype)
     if( !_buf.isContinuous() || _buf.empty() || _buf.cols*_buf.rows*_buf.elemSize() < bufSize )
         _buf.reserveBuffer(bufSize);
 
     uchar* buf = _buf.ptr();
     int i, j, dstep = (int)(img.step/sizeof(T));
+
+    // store labels and their corresponding region types
     int* labels = (int*)buf;
     buf += npixels*sizeof(labels[0]);
+    // store wavefront
     Point2s* wbuf = (Point2s*)buf;
     buf += npixels*sizeof(wbuf[0]);
+    // store region type
     uchar* rtype = (uchar*)buf;
+    buf += (npixels + 1) * sizeof(rtype[0]);
     int curlabel = 0;
 
-    // clear out label assignments
+    // clear out label and rtype buffers
     memset(labels, 0, npixels*sizeof(labels[0]));
+    memset(rtype, 0, (npixels + 1)*sizeof(rtype[0]));
 
     for( i = 0; i < height; i++ )
     {