Fix ring-builder crash.

If you adjust the replica count after adding devices but before the
first rebalance, the ring builder would crash.

To reproduce:

    $ swift-ring-builder crashy.builder create 8 3 1
    $ swift-ring-builder crashy.builder add r1z1-10.1.1.1:6000/sda1 1
    $ swift-ring-builder crashy.builder set_replicas 5
    $ swift-ring-builder crashy.builder rebalance

Change-Id: Id91cf8680961ccf6e3db153577e99cad545bee9d