Add null check before invoking onChallengeInterruptFinished

It's possible that the previous client's owner has died (binderDied),
which results in ClientMonitor clearing the listener.

Fixes: 171189072
Test: Start enrollment, kill settings, enter device credential.
      No crash observed.
Change-Id: I09f5d206b1641e7c4bc1cb0b8ec22045e9c9d35f