Call entity:destroy() too many times and get an error

[drhayes]

Here's the stacktrace from my game:

stack traceback:
	[string "boot.lua"]:637: in function '__newindex'
	lib/concord/list.lua:49: in function 'remove'
	lib/concord/instance.lua:76: in function 'flush'
	lib/concord/instance.lua:197: in function 'emit'
	game.lua:184: in function 'drawEntities'
	states/inWorld.lua:78: in function 'f'
	lib/gamera.lua:183: in function 'draw'
	core/camera.lua:65: in function 'draw'
	states/inWorld.lua:75: in function 'draw'
	main.lua:34: in function 'draw'
	[string "boot.lua"]:513: in function <[string "boot.lua"]:493>
	[C]: in function 'xpcall'```

I'm *pretty* sure what's happening is that one of my physics collision responders is removing something multiple times. If I add this line to `list.lua` inside `List:remove`:

```function List:remove(obj)
   local index = self.pointers[obj]
   if not index then return end -- this one's the new line!
   local size  = self.size

This error doesn't happen in my code anymore. Pretty sure that index comes back nil and makes things blow up.

Let me know if you need more info, can't repro, or it's totally something that's just my problem. I can also PR this up if you want.

[Keyslam]

Added your suggested line in the latest commit, hopefully that'll prevent it from happening again.
Thanks!