If we fail to enqueue a session because all threadpools are filled
we drop the session before we have rendered the clients address.

Report IP and port as "-" rather than passing NULL pointers, which
is not nice under any, and core dump solaris under some circumstances.

Fixes:  #598



git-svn-id: http://www.varnish-cache.org/svn/branches/2.1@5365 d4fa192b-c00b-0410-8231-f00ffab90ce4