Ranged attack bugs when targeting garrisoning, formation, or dead units

[historic_bruno]

There's a few bugs related to #18 / r11886, reproducible as follows:

• Error: a unit is fired upon while garrisoning. Ranged enemy units approach and attack some of your units. Meanwhile garrison your units in e.g. towers. If there's enough attackers and enough of your units, by chance a missile will be fired while your units are garrisoning, when it "hits" the entity has gone off-world (into the tower).
• Error: units are fired upon while in formation. Select several units and move them around an enemy ranged unit (towers work well) without stopping, until they get hit. This causes the null footprint error: the target entity is a "special/formation".
• Attack fails: a unit dies while being fired upon. To reproduce this it helps to have several towers with units garrisoned inside, firing upon numerous enemy units.

The first and third cases need a solution where the target calculation is independent of whether the target entity still exists (perhaps with a temporary entity created for that purpose as Philip originally suggested?) Otherwise we lose friendly fire and splash damage.

The second case should never happen, it's not possible for a player to target a formation but only a single entity in a formation. So something is going wrong with the target selection.