Allow random to work with 0.

[joeyhub]

This doesn't solve the problem of somehow you allow it to initiate with no contact points rather than noop or graceful error (with php-driver try to connect with host as empty string).

[joeyhub]

  const uint64_t limit = CASS_UINT64_MAX - CASS_UINT64_MAX % max;
  uint64_t r;
  do {
    r = rng_();
  } while (r >= limit);
  return r % max;

Return 0 might be wrong. Instead...

  const uint64_t limit = max ? CASS_UINT64_MAX - CASS_UINT64_MAX % max : CASS_UINT64_MAX;
  return max ? r % max : r;

[mpenick]

Created a story to fix the error handling around no contact points: https://datastax-oss.atlassian.net/browse/CPP-806