[MC-123771] Server crashes when an item frame with a map is loaded, and no player has that map in their inventory

The bug

The server crashes when a player connects to it.

Crash report

18w01a: crash-2018-01-03_14.47.58-server.txt

Description: Exception in server tick loop

java.lang.NullPointerException: Exception in server tick loop
	at hk.hashCode(SourceFile:121)
	at java.util.HashMap.hash(Unknown Source)
	at java.util.HashMap.remove(Unknown Source)
	at bjy.a(SourceFile:143)
	at rr.a(SourceFile:168)
	at rk.a(SourceFile:202)
	at net.minecraft.server.MinecraftServer.w(SourceFile:687)
	at qy.w(SourceFile:352)
	at net.minecraft.server.MinecraftServer.v(SourceFile:604)
	at net.minecraft.server.MinecraftServer.run(SourceFile:509)
	at java.lang.Thread.run(Unknown Source)