Fixes issue #8329 crash in NextHopConsistentHash where when the only (#8365)

host in a strategy is unavailable due to DNS lookup failures.

- refactored NextHopConsistentHash::findNextHop() to fix the crash
  and simplify it.
- fixed the unit tests in test_NextHopConsistentHash.cc so that test
  failure checks using strcmp() do not crash with a nullptr hostname
  result.

Author: jrushford