BUG/MINOR: resolvers: Fix lookup for a hostname in the state-file tree

BUG/MINOR: resolvers: Fix lookup for a hostname in the state-file tree

In resolv_check_response(), the condition 'if (!srvrq->named_servers)' was
inverted - it ran the named server lookup when the tree was empty/NULL
instead of when it was populated. This prevented proper hostname-based
matching of SRV records to servers from the state file.

The issue was introduced when tree of servers was replaced by a cebis tree
(fdf6fd5b45).

This patch must be backported to 3.3.