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.

Author: compnerd

CoreFoundation/Base.subproj/CFRuntime.c

@@ -1071,7 +1071,7 @@ CF_PRIVATE Boolean __CFProcessIsRestricted() {
 }
 
 #if DEPLOYMENT_TARGET_WINDOWS
-#define kNilPthreadT  { nil, nil }
+#define kNilPthreadT  INVALID_HANDLE_VALUE
 #else
 #define kNilPthreadT  (_CFThreadRef)0
 #endif