Fix github issue #213: Dropping unknown items crashes the game

The items will now just disappear when dropped.

builtin/item.lua

@@ -227,10 +227,12 @@ function minetest.item_drop(itemstack, dropper, pos)
 		local v = dropper:get_look_dir()
 		local p = {x=pos.x+v.x, y=pos.y+1.5+v.y, z=pos.z+v.z}
 		local obj = minetest.env:add_item(p, itemstack)
-		v.x = v.x*2
-		v.y = v.y*2 + 1
-		v.z = v.z*2
-		obj:setvelocity(v)
+		if obj then
+			v.x = v.x*2
+			v.y = v.y*2 + 1
+			v.z = v.z*2
+			obj:setvelocity(v)
+		end
 	else
 		minetest.env:add_item(pos, itemstack)
 	end