Make game::revive_corpse check if the spot for reviving is empty.

Prevents debug message that another zombie is already there.

Changed all calls to revive_corpse to check for the return value and handle non-reviving.

The debug message can be reproduced: kill several zombies (>10), put all their corpses on one square. Go away so the map with the corpses is outside of the reality bubble.
Wait 12 hours (or more, 48 is the most, all corpses older than 48 hours will revive immediately).
Go back to the corpses. Because they revive all at the same time, there is not enough time for them to spread and they will revive on top of each other. This causes the debug message.