Books: Limit the size of books

Really large books just waste hard drive space and the engine is not designed to
handle that much data in item metadata, this can cause strange things to happen.

mods/default/craftitems.lua

@@ -75,6 +75,7 @@ local function book_on_use(itemstack, user)
 	return itemstack
 end
 
+local max_text_size = 10000
 minetest.register_on_player_receive_fields(function(player, formname, fields)
 	if formname ~= "default:book" then return end
 	local inv = player:get_inventory()
@@ -103,8 +104,7 @@ minetest.register_on_player_receive_fields(function(player, formname, fields)
 		data.title = fields.title
 		data.owner = player:get_player_name()
 		data.description = "\""..fields.title.."\" by "..data.owner
-		data.text = fields.text
-		data.text_len = #data.text
+		data.text = fields.text:sub(1, max_text_size)
 		data.page = 1
 		data.page_max = math.ceil((#data.text:gsub("[^\n]", "") + 1) / lpp)