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/MacStd/common.hpp

@@ -323,6 +323,10 @@ namespace macstd
 			}
 		}
 
+	private:
+		// Force use of valid() method when testing handle.
+		operator bool() const;
+
 	protected:
 		T m_h; ///< Object handle
 	};