[extra_mobs] Fix critical attempt to index local c_pos (a nil value)

mods/ENTITIES/extra_mobs/piglin.lua

@@ -127,10 +127,12 @@ local piglin = {
 					self.state = "stand"
 				end
 				local c_pos = self.object:get_pos()
+				if c_pos then
 					self.what_traded = trading_items[math.random(#trading_items)]
 					for x = 1, math.random(self.what_traded.amount_min, self.what_traded.amount_max) do
 						minetest.add_item({x=math.random(c_pos.x - 1, c_pos.x + 1), y=math.random(c_pos.y - 1, c_pos.y + 1), z= math.random(c_pos.z - 1, c_pos.z + 1)}, self.what_traded.itemstring)
 					end
+				end
 			end)
 		end
 	end,