Nullify mPendingMO *before* updating the call state.

The code used to null out mPendingMO after it send out the notification
and updating it's state. While mPendingMO is not null, update-state code
will always short-circuit to offhook so it is necessary to reset the
variable before updating the state (GSM & CDMA already do this).

Bug: 23530363
Change-Id: I0491b371d0de52d21fd6e0d96e2660c63c38fada