Fix FOFs with transferline flag and many linedefs randomly crashing

Seems like linenum could go out of bounds if you use more than 4 linedefs for such a setup, therefore crashing the games sometimes while trying to retrieve the correct textures for the FOF This was more likely to happen in OpenGL for some reason crashing with following trace srb2win.RPT

I made a small test map for this, since it crashes seemingly randomly aswell as attach a windows build for testing too (based on current next) srb2win-mtferlinefix.exe tflinecrash.wad

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