Rename argument to priv check

Fixes crash when punching bones not owned by you, and may resolve https://github.com/minetest/minetest_game/issues/940

mods/bones/init.lua

@@ -5,7 +5,7 @@ bones = {}
 
 local function is_owner(pos, name)
 	local owner = minetest.get_meta(pos):get_string("owner")
-	if owner == "" or owner == name or minetest.check_player_privs(placer, "protection_bypass") then
+	if owner == "" or owner == name or minetest.check_player_privs(name, "protection_bypass") then
 		return true
 	end
 	return false