Fix crash with minecart on activator rail

mods/ENTITIES/mcl_minecarts/carts.lua

@@ -67,6 +67,8 @@ local function detach_driver(self)
 		--print("No player object found for "..driver_name)
 	end
 end
+mod.detach_driver = detach_driver
+
 function mod.kill_cart(staticdata, killer)
 	local pos
 	mcl_log("cart #"..staticdata.uuid.." was killed")

mods/ENTITIES/mcl_minecarts/carts/minecart.lua

@@ -7,7 +7,7 @@ local mod = mcl_minecarts
 local PASSENGER_ATTACH_POSITION = mod.PASSENGER_ATTACH_POSITION
 
 local function activate_normal_minecart(self)
-	detach_driver(self)
+	mod.detach_driver(self)
 
 	-- Detach passenger
 	if self._passenger then