Pocketmine-mp: Undefined property in Furnace

[zKoz210]

Issue description

Many errors associated with stoves

Steps to reproduce the issue

1. To go to the old world where there are furnace

OS and versions

• PocketMine-MP: https://github.com/pmmp/PocketMine-MP/commit/5190d9c1e28411e8de8bf77d769155b706d258d4
• PHP: 7.2
• Server OS: CentOS 7

Crashdump, backtrace or other files

[12:07:48] [Server thread/DEBUG]: #0 src/pocketmine/tile/Furnace(263): pocketmine\{closure}(integer 8, string Undefined property: pocketmine\nbt\tag\CompoundTag::$BurnTicks, string /home/hub/src/pocketmine/tile/Furnace.php, integer 263, array Array())
[12:07:48] [Server thread/DEBUG]: #1 src/pocketmine/level/Level(679): pocketmine\tile\Furnace->onUpdate()
[12:07:48] [Server thread/DEBUG]: #2 src/pocketmine/Server(2342): pocketmine\level\Level->doTick(integer 3612)
[12:07:48] [Server thread/DEBUG]: #3 src/pocketmine/Server(2497): pocketmine\Server->checkTickUpdates(integer 3612, double 1506244068.9248)
[12:07:48] [Server thread/DEBUG]: #4 src/pocketmine/Server(2238): pocketmine\Server->tick()
[12:07:48] [Server thread/DEBUG]: #5 src/pocketmine/Server(2117): pocketmine\Server->tickProcessor()
[12:07:48] [Server thread/DEBUG]: #6 src/pocketmine/Server(1699): pocketmine\Server->start()
[12:07:48] [Server thread/DEBUG]: #7 src/pocketmine/PocketMine(553): pocketmine\Server->__construct(BaseClassLoader object, pocketmine\utils\MainLogger object, string /home/hub/, string /home/hub/, string /home/hub/plugins/)

[dktapps]

_blames @Muqsit_

[dktapps]

Referenced commit should fix the issue, thanks!