Use localplayer to avoid crash

[Montandalar]

Under 5.5.0-dev, my client crashes back to the main menu with this stack trace:

Access denied. Reason: Lua: Runtime error from mod '' in callback on_item_use(): toolbuddy:init.lua:77: attempt to call field 'get_wielded_item' (a nil value)
stack traceback:
	toolbuddy:init.lua:77: in function 'checkTool'
	toolbuddy:init.lua:69: in function '?'
	*builtin*:client/register.lua:26: in function <*builtin*:client/register.lua:14>

Since minetest/1292bdbbcec45613c95aff9f2ea88aa49af25011, you need to use minetest.localplayer to call get_wielded_item as it was changed from a method on the minetest object to a method on the LocalPlayer userdata. This PR will update the CSM for 5.3 but would break backwards compatibility.