base: use `INVALID_HANDLE_VALUE` for the nil pthread
base: use `INVALID_HANDLE_VALUE` for the nil pthread

Windows does not use POSIX threads.  We emulate some of the behaviours.
The thread handle type on Windows is `HANDLE` rather than `pthread_t`.
Use the invalid handle instead to indicate that we do not have the value
yet.

parkera/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,apple/swift-corelibs-foundation,JGiola/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation