* Upon local mpls ecmp label deallocation, control-node would withdraw
  this mpls label in its path, if the label gets reused even before
  control-node retracts the label, due to quick activate and deactivate
  of the interface(oper state change), then mpls label in BGP path would
  point to internface NH, instead of composite NH as expected.
  Re-evaluation of BGP peer happens because it was one of the ecmp interface
  that got deactivated and added again.
  Add a check to verify that local ecmp mpls label is not deleted.
Unit test:
  Created ecmp with 2 instances, add the same route with aggregarte mpls
  label via bgp peer, deleted both interface so that route update is
  sent to BGP to withdraw this path, before BGP peer could withdraw
  same instance would be activated with aggregarate mpls label and
  route change would be triggered, verify that no crash is seen,
  and update of path happens fine
Closes-bug:#1381821

Change-Id: I27a0f8ca0f011cc4bab3de79787356d32563c4d6