login: add a missing error check for session_set_leader()

session_set_leader() may fail. If it fails, then manager_start_scope()
will trigger assertion.

This may be related to RHBZ#1663704.

(cherry picked from commit fe3ab84)

Related: #2158167

src/login/logind-dbus.c

@@ -784,7 +784,9 @@ static int method_create_session(sd_bus_message *message, void *userdata, sd_bus
                 goto fail;
 
         session_set_user(session, user);
-        session_set_leader(session, leader);
+        r = session_set_leader(session, leader);
+        if (r < 0)
+                goto fail;
 
         session->type = t;
         session->class = c;