Fix non-serializable item entity unload crash

Some items, like shulkers or books, can have so much metadata that the
corresponding item entity can not be serialized by the Minetest engine.

Without this patch, dropping such an item and then moving away crashes
Minetest, as it can not serialize the entity with serializeString16()
when unloading a map block.

The patch resets the overlong metadata of non-serializable item entities.
This avoids a crash and makes it possible to retrieve a “sanitized” item
without metadata when the mapblock containing the item entity is reloaded.

Originally sfan5 guessed the maximum possible item entity serialization size
that would not lead to a crash as 65530 bytes, but anon5 calculated it to be
actually 65487 bytes. This has been experimentally verified by erlehmann.

mods/ENTITIES/mcl_item_entity/init.lua

@@ -448,7 +448,7 @@ minetest.register_entity(":__builtin:item", {
 	end,
 
 	get_staticdata = function(self)
-		return minetest.serialize({
+		local data = minetest.serialize({
 			itemstring = self.itemstring,
 			always_collect = self.always_collect,
 			age = self.age,
@@ -456,6 +456,32 @@ minetest.register_entity(":__builtin:item", {
 			_flowing = self._flowing,
 			_removed = self._removed,
 		})
+		-- sfan5 guessed that the biggest serializable item
+		-- entity would have a size of 65530 bytes. This has
+		-- been experimentally verified to be still too large.
+		--
+		-- anon5 has calculated that the biggest serializable
+		-- item entity has a size of exactly 65487 bytes:
+		--
+		-- 1. serializeString16 can handle max. 65535 bytes.
+		-- 2. The following engine metadata is always saved:
+		--    • 1 byte (version)
+		--    • 2 byte (length prefix)
+		--    • 14 byte “__builtin:item”
+		--    • 4 byte (length prefix)
+		--    • 2 byte (health)
+		--    • 3 × 4 byte = 12 byte (position)
+		--    • 4 byte (yaw)
+		--    • 1 byte (version 2)
+		--    • 2 × 4 byte = 8 byte (pitch and roll)
+		-- 3. This leaves 65487 bytes for the serialization.
+		if #data > 65487 then -- would crash the engine
+			local stack = ItemStack(self.itemstring)
+			stack:get_meta():from_table(nil)
+			self.itemstring = stack:to_string()
+			return self:get_staticdata()
+		end
+		return data
 	end,
 
 	on_activate = function(self, staticdata, dtime_s)