ogs_assert on enb_ue can fail.

[spencersevilla]

The line "ogs_assert(enb_ue);" in s1ap-handler.c:469 sometimes fails and crashes the mme. Failure in this case means that we cannot find the enb_ue ID given to us by the eNB in the ContextSetupResponse message. Given that this variable is given to us by the network, we should not assert on its absence.

In terms of logic, I am not sure yet why this assert failed, since we ostensibly just created this enb_ue value in mme-context.c:2033. Could be a race condition, will dig further, but my point about asserts on network values still stands.