(fixed) Size 2 monster bugs.

[Joramun]

First : when a monster of size 2 is not in the center of a tile, it is possible to pick up items from the corners of its tile (even if it is over the items).

Second :
LUA ERROR: Lua Function sys_ai_near: base/monster_ai.lua:783: dsb_reposition: Invalid size2 subtile 3
Approximately happened (in CSB first room) when a worm spawned on or came near another worm (or possibly on the same tile, but without teaming up)