Fix nil index crash in update_item_count_settings

[sparr]

When another mod changes a global mod setting, event.player_index will be nil and this function will crash.

e.g. this happens:

The mod Project Cybersyn (1.2.16) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event cybersyn::on_init()
The mod Picker Inventory Tools (1.1.15) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event PickerInventoryTools::on_runtime_mod_setting_changed (ID 61)
__stdlib__/stdlib/event/event.lua:368: __PickerInventoryTools__/scripts/item-count.lua:43: bad argument #3 of 3 to '__index' (string expected, got nil)
stack traceback:
    [C]: in function 'error'
    __stdlib__/stdlib/event/event.lua:368: in function 'dispatch_event'
    __stdlib__/stdlib/event/event.lua:438: in function <__stdlib__/stdlib/event/event.lua:396>
stack traceback:
    [C]: in function '__newindex'
    __cybersyn__/scripts/main.lua:940: in function <__cybersyn__/scripts/main.lua:937>

when Cybersyn does this:

local setting = settings.global["cybersyn-invert-sign"]
setting.value = false
settings.global["cybersyn-invert-sign"] = setting