Fix bots walking off cliffs accidently

phys_maxbarrier in ai_move.c was changed to get value from botlib in commit 864c08a04c94dc57ca533cb763ae7b5f964085aa.
However it hadn't been setup in botlib yet, so I've moved it to after botlib loads map and sets up physics libvars.

The walking off cliffs issue was caused by BotGapDistance failing to detect gaps when phys_maxbarrier variable in ai_main.c is 0.