Consider the exit family when applying guard restrictions.

When the new path selection logic went into place, I accidentally
dropped the code that considered the _family_ of the exit node when
deciding if the guard was usable, and we didn't catch that during
code review.

This patch makes the guard_restriction_t code consider the exit
family as well, and adds some (hopefully redundant) checks for the
case where we lack a node_t for a guard but we have a bridge_info_t
for it.

Fixes bug 22753; bugfix on 0.3.0.1-alpha. Tracked as TROVE-2016-006
and CVE-2017-0377.
node_exit_policy_is_exact(const node_t *node, sa_family_t family)
{
  if (family == AF_UNSPEC) {
    return 1; /* Rejecting an address but not telling us what address
               * is a bad sign. */
  } else if (family == AF_INET) {
    return node->ri != NULL;
  } else if (family == AF_INET6) {
    return 0;
  }
  tor_fragile_assert();
  return 1;
}

node_exit_policy_is_exact(const node_t *node, sa_family_t family)
{
  if (family == AF_UNSPEC) {
    return 1; /* Rejecting an address but not telling us what address
               * is a bad sign. */
  } else if (family == AF_INET) {
    return node->ri != NULL;
  } else if (family == AF_INET6) {
    return 0;
  }
  tor_fragile_assert();
  return 1;
}
