Fix a bug in Xpro making the game crash when xp_atual is nil

TODO

@@ -11,4 +11,5 @@ minetest-solebull - LGPL-2.1 - A PVP/faction game for minetest based on Cobalt.
    
    
 * v0.0.2-5 (02 Jan. 2019 - ???) CLOC ???,???
+- Fix a bug in Xpro making the game crash when xp_atual is nil
 - Add more mc mobs with XPs

mods/xpro/api.lua

@@ -110,6 +110,13 @@ xpro.add_xp = function(name, xp_add)
 	local tb_name = "jogador_"..name
 	
 	local xp_atual = xpro.get_player_xp(name)
+
+	-- In rare case, xp_atual can be nil. So, better than nothing
+	-- do not let the game crash
+	if xp_atual == nil then
+	   minetest.log("warning", "xp_atual is nil. Will not add XPs")
+	   return
+	end
 	
 	-- Evita salvar xp negativo
 	local new_xp = xp_atual+xp_add