P_FindSpecialLineFromTag crash fix

This fixes the case where an in-level sector or FOF control sector with a tag of -1 or 65535 (seriously, who does that?) can possibly lead to a crash in this function when searching for heat linedef specials in 1st person. And also the same kind of crash any other sort of weird case where the tag number that function uses for searching is -1 or 65535, I suppose!