Prevent some mobs from spawning in nullspace

__Killer tomatoes :__
* Prevent killer tomato from spawning inside nullspace
* It's now impossible to eat an awakening killer tomato
* Fixed been able to spawn lots of killer tomatoes by spam-clicking the tomato object

__Drones :__

Prevent an edge case which would lead to a client spawning as drone in null space (may happen when two ghosts clients clicked on a drone before accepting : the first one was correctly respawn, the second was send to nullspace)

__Brains :__

Makes brainmobs qdel instead of piling up inside the brain object.
I haven't been able to reproduce the mobbrain null location bug, but i'm pretty sure this didn't help.

Fixes #5785 (except _maybe_ mobbrains...)