Don't crash if a system restore fails before constructing the PMBA

If a whole-system restore operation failed at just the wrong point,
we'd wind up in the teardown code without a certain vital bit of it
having been initialized, and crash on the null pointer.  Now we
recognize this failure mode and make sure not to do that.

Bug 18574450

Change-Id: Ifa2c10ce16bb3c6bc916ed7151c5fd51b7225691