Player cannot get frostnip if s/he is comfortable

Hardcoded a boundry on frostbite. I think it was technically possible to be in such cold weather that, despite being comfortable bodytemp wise, you would still have gotten frostnip/bite.

This is in response to this forum post : http://smf.cataclysmdda.com/index.php?topic=5877.msg183286#msg183286