Protect against NPE for ActivityRecords without a stack.

A previous change allowed us to remove stack that no longer contained
any task. This was causing some NPE when an ActivityRecord.Token or
some other cached ActivityRecord later gets converted back to an
ActivityRecord and we try to access its stack.

Bug: 19552874
Change-Id: Ie9454bbce56591b337f97af40f8c00b8597becdf