Fix FOFs which use transferline flag and many linedefs randomly crashing

linenum could go out of bounds if you use more than 4 linedefs for such setup, hence making the game unable to retrieve textures and therefore crashing the games at random

can be observed in https://mb.srb2.org/addons/bionicle-sky-temple.2269/

many thx to indev for helping me figure this one out c: