[crash] game crashes when loading spawners with custom NBT

Put the summary of the bug you're having here

What I expected to happen was...:
Get on my world with cutsom spawners

What actually happened was...:
The game crashed when my spawners loaded

Steps to Reproduce:*
1. make a spawner with custom NBT data in McEdit
2. Get on the world
3. The game will crash when the spawner is loaded