Common: Enforce explicit handle validation

Some invalid handle values are -1 or INVALID_HANDLE_VALUE, hence our
handle template has a specific invalid handle value parameter we should
always test against. Time and again, I find my code simply comparing
handle against zero.

Signed-off-by: Simon Rozman <simon@rozman.si>

include/WinStd/Common.h

@@ -914,6 +914,9 @@ namespace winstd
         handle(_In_ const handle<handle_type, INVAL> &h) noexcept {};
         handle<handle_type, INVAL>& operator=(_In_ const handle<handle_type, INVAL> &h) noexcept {};
 
+        // Force use of valid() method when testing handle.
+        operator bool() const;
+
     public:
         ///
         /// Attaches already available object handle.