thread: fix segfault on Windows 64 bits

`lpExitCode` is a pointer to a long. A long is 32 bits wide on Windows.

It means that on Windows 64bits, `GetExitCodeThread()` doesn't set/clear the high-order bytes of the 64 bits memory space pointed at by `value_ptr`.
Merge pull request #1622 from yorah/fix/thread-segfault

thread: fix segfault on Windows 64 bits