Fix no-formation orders after several formation orders.

When giving several queued formation-walk orders, then a queued non-formation order (such as gathering), the gather order would be silently dropped. This did not happen if only one formation-walk order was given, which is why this issue was not noticed in rP24480

To fix it, explicitly only drop pre-"leave formation" orders in the queue for formation members.

Reported By: faction02

Differential Revision: https://code.wildfiregames.com/D3550