BUG/MINOR: backend: assign the wait list after the error check

Commit 85b73e9 ("BUG/MEDIUM: stream: Make sure polling is right on retry.")
introduced a possible null dereference on the error path detected by gcc-7.
Let's simply assign srv_conn after checking the error and not before.

No backport is needed.
diff --git a/src/backend.c b/src/backend.c
index 834caec..4d75df6 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1167,12 +1167,13 @@
 		LIST_DEL(&srv_conn->list);
 		LIST_INIT(&srv_conn->list);
 	}
-	srv_conn->send_wait = send_wait;
-	srv_conn->recv_wait = recv_wait;
 
 	if (!srv_cs)
 		return SF_ERR_RESOURCE;
 
+	srv_conn->send_wait = send_wait;
+	srv_conn->recv_wait = recv_wait;
+
 	if (!(s->flags & SF_ADDR_SET)) {
 		err = assign_server_address(s);
 		if (err != SRV_STATUS_OK)