[BUG] log: retrieve the target from the session, not the SI

Since we now have the copy of the target in the session, use it instead
of relying on the SI for it. The SI drops the target upon unregister()
so applets such as stats were logged as "NOSRV".
diff --git a/src/backend.c b/src/backend.c
index 397216d..4b923a7 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -969,6 +969,7 @@
 	 */
 	stream_sock_prepare_interface(s->req->cons);
 	s->req->cons->connect = tcp_connect_server;
+	/* the target was only on the session, assign it to the SI now */
 	copy_target(&s->req->cons->target, &s->target);
 
 	/* process the case where the server requires the PROXY protocol to be sent */