resolves #10548 - `FLANN::knnSearch` garbage bug (when kNN is larger than the dataset size)

modules/flann/src/miniflann.cpp

@@ -482,6 +482,9 @@ void runKnnSearch_(void* index, const Mat& query, Mat& indices, Mat& dists,
     typedef typename Distance::ResultType DistanceType;
     int type = DataType<ElementType>::type;
     int dtype = DataType<DistanceType>::type;
+    IndexType* index_ = (IndexType*)index;
+
+    CV_Assert((size_t)knn <= index_->size());
     CV_Assert(query.type() == type && indices.type() == CV_32S && dists.type() == dtype);
     CV_Assert(query.isContinuous() && indices.isContinuous() && dists.isContinuous());
 
@@ -489,8 +492,8 @@ void runKnnSearch_(void* index, const Mat& query, Mat& indices, Mat& dists,
     ::cvflann::Matrix<int> _indices(indices.ptr<int>(), indices.rows, indices.cols);
     ::cvflann::Matrix<DistanceType> _dists(dists.ptr<DistanceType>(), dists.rows, dists.cols);
 
-    ((IndexType*)index)->knnSearch(_query, _indices, _dists, knn,
-                                   (const ::cvflann::SearchParams&)get_params(params));
+    index_->knnSearch(_query, _indices, _dists, knn,
+                      (const ::cvflann::SearchParams&)get_params(params));
 }
 
 template<typename Distance>