Fix error in expr call when there is only one node
Fix error in expr call when there is only one node

Change-Id: Ic963df930bb5db368457d53aaa3d8691a28b121f

gkatsikas/onos,gkatsikas/onos,gkatsikas/onos,oplinkoms/onos,opennetworkinglab/onos,opennetworkinglab/onos,gkatsikas/onos,opennetworkinglab/onos,oplinkoms/onos,oplinkoms/onos,opennetworkinglab/onos,oplinkoms/onos,gkatsikas/onos,opennetworkinglab/onos,opennetworkinglab/onos,oplinkoms/onos,oplinkoms/onos,gkatsikas/onos,oplinkoms/onos