Do not attempt to xmlReadFile() when the file does not exists.
Do not attempt to xmlReadFile() when the file does not exists.

That way we can avoid non solicited stderr message from
libxml. Note that we need to stat the file anyway.

Addressing:
I/O warning : failed to load external entity

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