Security: Use explicit handle validation

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

include/MacStd/Security.hpp

@@ -61,7 +61,7 @@ namespace macstd
 				case errAuthorizationInteractionNotAllowed: return "User interaction not allowed for authorization";
 			}
 			cfstring cfstr(SecCopyErrorMessageString(num, NULL));
-			if (!cfstr)
+			if (!cfstr.valid())
 				return "Security Framework error " + std::to_string(num);
 			auto ptr = CFStringGetCStringPtr(cfstr, kCFStringEncodingUTF8);
 			if (!ptr)