Fix crash when measuring storage.

If we are measuring storage and a volume is removed at the same time,
we can enter a state where we previously verified the volume existed,
but it no longer does. This causes an NPE.

By adding in a null check, we can avoid this crash.

Change-Id: Ib8dbf05102a122bdf4bb6063374e993a1de68425
Fixes: 36689190
Test: None