fix crash if curved rail has no items_to_place_this

[daviscook477]

when checking "straight-rail" there is a check on "proto.items_to_place_this" before then trying to use pairs on "proto.items_to_place_this"

however when checking "curved-rail" there is no check on "proto.items_to_place_this" before then trying to use pairs on "proto.items_to_place_this"

this means that when another mod adds a type of rail for which there is no item to place it, FARL crashes with the following error:

Please report this error to the mod author.

Error while running event FARL::on_init()
__FARL__/control.lua:79: bad argument #1 of 2 to 'pairs' (table expected, got nil)
stack traceback:
    [C]: in function 'pairs'
    __FARL__/control.lua:79: in function 'getRailTypes'
    __FARL__/control.lua:228: in function <__FARL__/control.lua:222>

this PR adds the missing check to the "curved-rail" line