Handle NULL flow in FLOW_LOCK.
Handle NULL flow in FLOW_LOCK.

Change-Id: I8a202aca7dc7a7fd1bf86509b3509697b5e51f62
Closes-bug: #1552203

tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller,tcpcloud/contrail-controller