Fix crash in swift-ring-builder's list_parts command.

If you run list_parts against a builder that has never been
rebalanced, you'd get a crash. Now you don't.

To reproduce:

$ swift-ring-builder foo.builder create 8 3 1
$ swift-ring-builder foo.builder add r1z1-1.2.3.4:6000/sda 100
$ swift-ring-builder foo.builder list_parts z1

Change-Id: Ic3edffab0c5c2e9551a2f89ddb881153f0b07db7
Merge "Fix crash in swift-ring-builder's list_parts command."