Fix a mutex deadlock

internal/multiplex/switchboard.go

@@ -106,7 +106,9 @@ func (sb *switchboard) removeConn(closing *connEnclave) {
 	}
 	if len(sb.ces) == 0 {
 		sb.session.SetTerminalMsg("no underlying connection left")
+		sb.cesM.Unlock()
 		sb.session.Close()
+		return
 	}
 	sb.cesM.Unlock()
 }