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.
diff --git a/src/session.c b/src/session.c
index 3f117e6..0a6b130 100644
--- a/src/session.c
+++ b/src/session.c
@@ -2202,7 +2202,7 @@
 				/* check for HTTP mode and proxy server_name_hdr_name != NULL */
 				if ((s->flags & SN_BE_ASSIGNED) &&
 				    (s->be->mode == PR_MODE_HTTP) &&
-				    (s->be->server_id_hdr_name != NULL)) {
+				    (s->be->server_id_hdr_name != NULL && s->target)) {
 					http_send_name_header(&s->txn, s->be, objt_server(s->target)->id);
 				}
 			}