pf: cope with missing rpool.cur

If we're evaluating a pfsync'd state (and have different rules on both
ends) our state may point to the default rule, which does not have
rpool.cur set. As a result we can end up dereferencing a NULL pointer.

Explicitly check for this when we try to re-construct the route-to interface.

Also add a test case which can trigger this issue.

MFC after: 3 days
See also: https://redmine.pfsense.org/issues/14804
Sponsored by: Rubicon Communications, LLC ("Netgate")