winpr: CloseHandle did not release the thread TCB

This resulted in huge memory leaks - 8MB per thread, depending on
the system's default stack size.
The leak happend even if CloseHandle() was correctly used to "detach"
the thread but WaitForSingleObject was never called.
Merge pull request #1955 from nfedera/fix-2014-07-10-02

winpr: CloseHandle did not release the thread TCB