Prevent badarith when memory is unavailable.

When a node becomes reachable directly after starting and memsup is
still starting, memory can be unavailable. Prevent this computation
until it's available, avoding crashing webmachine with a badarith.

This also causes problems with incompatible, but reachable, nodes
throwing the badarith exception.
Merge pull request #121 from basho/csm-backport-lowmemfix-1.3

Prevent badarith when memory is unavailable.