Fix AI bug in passive_leader_shares_keep candidate action. When the AI leader moves off the keep for an ally leader and doesn't have full moves left, it was previously not checked that the goal hex is within reach of the leader.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56028 75b3775d-3801-0410-9e12-e2e43121521d