Random crash when computing goals

[hwoarangmy]

I've had several times a crash within Seat::checkAllGoals.

After a bit of search, it seems a vector iterator can be invalidated when the vector is resized (which might happen when elements are inserted). That might crash.
This one was tricky for me as I'm not used to STL containers :)

If interested, I'm talking about :
https://github.com/OpenDungeons/OpenDungeons/blob/development/source/Seat.cpp#L124
then we do
https://github.com/OpenDungeons/OpenDungeons/blob/development/source/Seat.cpp#L126