[MC-5672] Crash when trying to convert an old world

Minecraft crashes when trying to convert Skylands 1.3.

The map can be downloaded from here : http://www.minecraftforum.net/topic/35387-survskylands-13-custom-world-thats-entirely-abandoned/

A crash report is attached.

Description: Reading NBT data
java.lang.ClassCastException: bx cannot be cast to bz
	at bq.f(SourceFile:134)
	at aah.a(SourceFile:30)
	at ahs.a(SourceFile:210)
	at ahs.a(SourceFile:181)
	at ahs.a(SourceFile:140)
	at net.minecraft.server.MinecraftServer.b(SourceFile:394)
	at bdz.a(SourceFile:54)
	at bdz.c(SourceFile:99)
	at net.minecraft.server.MinecraftServer.run(SourceFile:588)
	at fy.run(SourceFile:849)