Found via `codespell -q 3 -I ../opencv-whitelist.txt --skip="./3rdparty"`

- check result from CV_XADD() directly
- decrease urefcount after unmap() call only

Changed the type of variable *r* from int to size_t.
This change makes sure that a valid result of std::max(r + delta,
(r*3+1)/2) can be passed into the reserve function.

to avoid silent crashes of OpenCL worker threads.