core: throw yyerror on invalid cfg var definition

(cherry picked from commit efe6966fa1691cbb762ec13aa11164b0989e3cb4)

src/core/cfg.y

@@ -1578,6 +1578,7 @@ cfg_var_idn: ID
 	| NUMBER {
 		yyerror("cfg var field name - use of number or reserved token not allowed: %s",
 				yy_number_str);
+		YYERROR;
 	}
 	;