BUG/MAJOR: http: don't emit the send-name-header when no server is available

Lukas Benes reported that http-send-name-header causes a segfault if no
server is available because we're dereferencing the session's target which
is NULL. The tiniest reproducer looks like this :

     listen foo
         bind :1234
         mode http
         http-send-name-header srv

This obvious fix must be backported to 1.4 which is affected as well.