Refactor: Extract function: oval_variable_validate_ext_var
Refactor: Extract function: oval_variable_validate_ext_var

This change also introduces SYSCHAR_FLAG_ERROR when the values
are empty.

ybznek/openscap,ybznek/openscap,postfix/openscap,OpenSCAP/openscap,redhatrises/openscap,mpreisler/openscap,mpreisler/openscap,openprivacy/openscap,ybznek/openscap,redhatrises/openscap,jan-cerny/openscap,postfix/openscap,Hexadorsimal/openscap,jan-cerny/openscap,mpreisler/openscap,ybznek/openscap,openprivacy/openscap,mpreisler/openscap,OpenSCAP/openscap,openprivacy/openscap,OpenSCAP/openscap,redhatrises/openscap,jan-cerny/openscap,Hexadorsimal/openscap,postfix/openscap,redhatrises/openscap,redhatrises/openscap,redhatrises/openscap,openprivacy/openscap,jan-cerny/openscap,postfix/openscap,openprivacy/openscap,Hexadorsimal/openscap,Hexadorsimal/openscap,ybznek/openscap,OpenSCAP/openscap,OpenSCAP/openscap,jan-cerny/openscap,jan-cerny/openscap,postfix/openscap,postfix/openscap,Hexadorsimal/openscap,OpenSCAP/openscap,ybznek/openscap,mpreisler/openscap,openprivacy/openscap,Hexadorsimal/openscap,mpreisler/openscap