Fix 'error: ‘%s’ directive argument is null [-Werror=format-overflow=]' in assertions.c

(cherry picked from commit f4260dc0c5)

lib/isc/assertions.c

@@ -82,7 +82,7 @@ isc_assertion_typetotext(isc_assertiontype_t type) {
 		result = "INVARIANT";
 		break;
 	default:
-		result = NULL;
+		result = "UNKNOWN";
 	}
 	return (result);
 }