Merge pull request #24779 from MaximSmolskiy:fix-bug-in-ChessBoardDetector-findQuadNeighbor

Fix bug in ChessBoardDetector::findQuadNeighbors #24779

### Pull Request Readiness Checklist

`corners` and `neighbors` indices means not filling order, but relative position. So, for example if `quad->count = 2`, it doesn't mean that `quad->neighbors[0]` and `quad->neighbors[1]` are filled. And we should should iterate over all four `neighbors`.

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/calib3d/src/calibinit.cpp

@@ -156,7 +156,8 @@ struct ChessBoardQuad
     float edge_len; // quad edge len, in pix^2
     // neighbors and corners are synced, i.e., neighbor 0 shares corner 0
     ChessBoardCorner *corners[4]; // Coordinates of quad corners
-    struct ChessBoardQuad *neighbors[4]; // Pointers of quad neighbors
+    struct ChessBoardQuad *neighbors[4]; // Pointers of quad neighbors. M.b. sparse.
+    // Each neighbors element corresponds to quad corner, but not just sequential index.
 
     ChessBoardQuad(int group_idx_ = -1) :
         count(0),
@@ -1701,12 +1702,12 @@ void ChessBoardDetector::findQuadNeighbors()
                     continue;
 
                 // Check that each corner is a neighbor of different quads
-                for(j = 0; j < closest_quad->count; j++ )
+                for(j = 0; j < 4; j++ )
                 {
                     if (closest_quad->neighbors[j] == &cur_quad)
                         break;
                 }
-                if (j < closest_quad->count)
+                if (j < 4)
                     continue;
 
                 // check whether the closest corner to closest_corner