Fixed a memory access error in CV_Remap_Test::generate_test_data.

begin_x[1] is not the second component of the element, but the element
after the one pointed to begin_x. When begin_x points to the last
element, that line overwrites data past the end of the allocation, which,
during my tests, happened to contain the reference count for the matrix.
Hilarity ensues.

modules/imgproc/test/test_imgwarp_strict.cpp

@@ -679,8 +679,8 @@ void CV_Remap_Test::generate_test_data()
             MatIterator_<Vec2s> begin_x = mapx.begin<Vec2s>(), end_x = mapx.end<Vec2s>();
             for ( ; begin_x != end_x; ++begin_x)
             {
-                begin_x[0] = static_cast<short>(rng.uniform(static_cast<int>(_n), std::max(src.cols + n - 1, 0)));
-                begin_x[1] = static_cast<short>(rng.uniform(static_cast<int>(_n), std::max(src.rows + n - 1, 0)));
+                (*begin_x)[0] = static_cast<short>(rng.uniform(static_cast<int>(_n), std::max(src.cols + n - 1, 0)));
+                (*begin_x)[1] = static_cast<short>(rng.uniform(static_cast<int>(_n), std::max(src.rows + n - 1, 0)));
             }
 
             if (interpolation != INTER_NEAREST)