util_sock: Fix memcache bug in get_mydnsfullname.

get_mydnsfullname relied on memcache_add(); memcache_lookup() working.
When run from ntlm_auth, the global_cache variable in memcache is NULL, so
the add and lookup both fail. In that case, just return the result of the
getaddrinfo call.

Jeremy, please check.
(This used to be commit 1db41ff52565e9f336a22fb9ffd80d51677e023b)