Do not wipe last_fqdn out in each iteration

Addressing coverity findings:
1. openscap-1.1.0/src/DS/rds.c:366:assignment – Assigning: "last_fqdn" = "NULL".
2. openscap-1.1.0/src/DS/rds.c:374:null – At condition "last_fqdn", the value of "last_fqdn" must be NULL.
3. openscap-1.1.0/src/DS/rds.c:374:dead_error_condition – The condition "!last_fqdn" must be true.
4. openscap-1.1.0/src/DS/rds.c:378:dead_error_line – Execution cannot reach this statement "xmlAddNextSibling(last_fqdn...".