Fix a nil digger bug in xpro

ROADMAP

@@ -259,6 +259,12 @@ It's really fast. May be used on a website, to show the actual map.
 **** Try mumble-link
 - see https://github.com/chipgw/minetest-mumble-wrapper
 
+**** Some crashes
+- [ ] 2019-01-15 17:35:29: ERROR[Main]: ServerError: AsyncErr: ServerThread::run Lua: Runtime error from mod '*builtin*' in callback luaentity_Step(): ...netest/games/minetest-pvp/mods/xpro/metodos/dig_node.lua:21: attempt to index local 'digger' (a nil value)
+2019-01-15 17:35:29: ERROR[Main]: stack traceback:
+2019-01-15 17:35:29: ERROR[Main]: 	...netest/games/minetest-pvp/mods/xpro/metodos/dig_node.lua:21: in function 'callback'
+2019-01-15 17:35:29: ERROR[Main]: 	/usr/local/share/minetest/builtin/game/falling.lua:103: in function </usr/local/share/minetest/builtin/game/falling.lua:53>
+
 **** DONE Another server fails
      *These ones are important : the game is broken*
 - [X] Another villagers spawn-related

mods/xpro/metodos/dig_node.lua

@@ -14,15 +14,19 @@ xpro.dig_node_xp_list = {}
 
 -- Chamada global
 minetest.register_on_dignode(function(pos, oldnode, digger)
-	
-	-- Verifica se node gera XP
-	if xpro.dig_node_xp_list[oldnode.name] then
-		if xpro.dig_node_xp_list[oldnode.name] > 0 then
-			xpro.add_xp(digger:get_player_name(), xpro.dig_node_xp_list[oldnode.name])
-		else
-			xpro.rem_xp(digger:get_player_name(), math.abs(xpro.dig_node_xp_list[oldnode.name]))
-		end
-	end
+
+      if digger == nil then
+	 return
+      end
+      
+      -- Verifica se node gera XP
+      if xpro.dig_node_xp_list[oldnode.name] then
+	 if xpro.dig_node_xp_list[oldnode.name] > 0 then
+	    xpro.add_xp(digger:get_player_name(), xpro.dig_node_xp_list[oldnode.name])
+	 else
+	    xpro.rem_xp(digger:get_player_name(), math.abs(xpro.dig_node_xp_list[oldnode.name]))
+	 end
+      end
 end)
 
 -- Registrar item para o evento