Do not bombard a tile with only unreachable units
This is probably a bug worth backporting since nobody actually wants to do it. E.g. I wanted to crash a stack of enemy helicopters bombing them with mine under hp_loss_pct - oops, it's not possible, they are inaccessible for each other, but my one already loses all the hits. In later versions, should be enforced if you see all vision layers on the target tile.