Cleanup pending MO call when it disconnects before alerting.

Ensure that where a call is in the pending MO state and gets
disconnected prior to alerting that it will be cleaned up so that other
calls can be placed.

Test: Wrote new unit test for this case.
Fixes: 250229749
Change-Id: I4daa33915fcc3fa5b128175c19827fc332c05d1d