BUG/MEDIUM: server: segv when adding server with hostname from CLI

BUG/MEDIUM: server: segv when adding server with hostname from CLI

When calling 'add server' with a hostname from the cli (runtime),
str2sa_range() does not resolve hostname because it is purposely
called without PA_O_RESOLVE flag.

This leads to 'srv->addr_node.key' being NULL. According to Willy it
is fine behavior, as long as we handle it properly, and is already
handled like this in srv_set_addr_desc().

This patch fixes GH #1865 by adding an extra check before inserting
'srv->addr_node' into 'be->used_server_addr'. Insertion and removal
will be skipped if 'addr_node.key' is NULL.

It must be backported to 2.6 and 2.5 only.