Reset client_state toi_mode to m_undefined after RSU

Client state end_rsu() didn't reset toi_mode to m_undefined,
which caused an assertion when NBO was started after RSU.

As a fix, reset toi_mode to m_undefined in end_rsu() after
changing mode.

src/client_state.cpp

@@ -554,6 +554,7 @@ int wsrep::client_state::end_rsu()
     }
     wsrep::unique_lock<wsrep::mutex> lock(mutex_);
     mode(lock, toi_mode_);
+    toi_mode_ = m_undefined;
     return ret;
 }