The index of valid elements in vector will change when deleting vector elements.

In twice_nat and snat flow, vpp will alloc outside address and port for session, and save the outside address index in session. When delete one (dynamic)session, vpp will free the port of outside address for the session. 

But now, the outside address index which saved in session may be wrong because of deleting other outside address before. 

This problem will cause vpp to thrown an exception.