optimises return_thermal_protection

Created by: DrCelt

sorry @Clusterfack The reason it was causing any lag is because every tick of life may or may not have ran a loop through every piece of clothing determining what body parts it covered and gave an appropriate value. This means a piece of clothing will only be ran through once, saved in the global variable, and any piece of clothing which has the same body_parts_covered variable will take that value too. Hence, optimised.