Crashing when pressing tab while typing a command.

The client crashes when pressing tab after typing "/scoreboard players list", when it's supposed to write a player name.

This happens in both survival and creative if you got cheat mode on.

*What I expected to happen was...: After "list " a player name (a player that have any value in the list, eg a score of 10 on a dummy) should be added, so it will say "/scoreboard players list PlayerName" instead of a crash.

*What actually happened was...: As a redstoner I knew from previous versions of Minecraft that pressing tab will automatically continue typing on a command for you. So, as lazy as I am, I typed "/scoreboard " then pressed tab, which changed the text to "/scoreboard players " then again, so it's now "scoreboard players list ". When I now pressed tab it crashed when it's supposed to write my name. I tested this in the three newest snapshots and this resulted the same way in all of them, but worked fine in the official release of 1.5.2.

Steps to Reproduce:
1. Type "/scoreboard" with a space after the word.
2. Press tab until it writes "players" after "/scoreboard". It will now say "/scoreboard players". Add a space after "players".
3. Press tab until it writes "list" after "/scoreboard players". It will now say "/scoreboard players list". Add a space after "list".
4 Press tab again, this time the client crashes.