BUG/MINOR: server: better handling of OOM in srv_set_fqdn()

BUG/MINOR: server: better handling of OOM in srv_set_fqdn()

This function may face an OOM on strdup() in the middle of the hostname
or hostname_dn replacement, leaving NULLs in either or both of the server's
fields, which is definitely not good for other call places.

Let's perform a safe replacement instead: we first allocate the new
values, and only if they are successful, then we release the previous
ones and replace them.

It is not necessary to backport this unless the issue is reported (it
was found via code review).