[Nighly build] Forest crash

Several times on last week I got a crash while walking through a forest.
It happens at random and there is no obvious way to reproduce it. I caught it in debugger - for some reason it can’t ++ the iterator.
Upd. Curent last commit hash: 235cf594e9b8503c068637d5b8d47b96b48e1ff7

We’ve gotten a ton of these mon_in_field crashes lately, caused by bad pointers from wherever.