BUG/MAJOR: namespaces: conn->target is not necessarily a server

BUG/MAJOR: namespaces: conn->target is not necessarily a server

create_server_socket() used to dereference objt_server(conn->target),
but if the target is not a server (eg: a proxy) then it's NULL and we
get a segfault. This can be reproduced with a proxy using "dispatch"
with no server, even when namespaces are disabled, because that code
is not #ifdef'd. The fix consists in first checking if the target is
a server.

This fix does not need to be backported, this is 1.6-only.