[MC-14126] Binding "sneak" to any mouse button and trying to mount an entity crashes the game

Not too sure what went wrong, other than that I mounted the horse (saddled) and opened my inventory at almost the same time. The client crashed and died instantaneously (no error screen) but didn't stop the 'Java.exe' process, which retained it's memory (the process had to be closed manually, else the launcher refused to start a new client due to the process already being in use).

When I attempted to go back into the world, after the loading screen (after 'Downloading World' prompt), it instantly crashed, exiting completely out of the client without an error screen, and the Java.exe process was still running again.

This output from the launcher may be relevant to the issue, as it seems to recall some information from the moment of the crash;

Client> 2013-04-20 01:14:10 [SERVER] [INFO] Preparing start region for level 0
Client> 2013-04-20 01:14:11 [SERVER] [INFO] Preparing spawn area: 99%
Client> loading single player
Client> 2013-04-20 01:14:11 [SERVER] [INFO] sebbeshs[/127.0.0.1:0] logged in with entity id 248 at (385.92508199461025, 66.7000000178814, -238.48337619840805)
Client> java.io.FileNotFoundException: http://assets.minecraft.net/1_6_has_been_released.flag
Client> 	at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
Client> 	at kq.a(SourceFile:237)
Client> 	at bhq.run(SourceFile:114)
Client> 	at java.lang.Thread.run(Unknown Source)
Client> 2013-04-20 01:14:12 [SERVER] [INFO] Saving and pausing game...
Client> 2013-04-20 01:14:12 [SERVER] [INFO] Saving chunks for level '[13w16a] ADVENTURE!!!'/Overworld
Client> u: Ticking screen
Client> 	at asm.j(SourceFile:1101)
Client> 	at asm.N(SourceFile:566)
Client> 	at asm.c(SourceFile:522)
Client> 	at net.minecraft.client.main.Main.main(SourceFile:78)
Client> Caused by: java.lang.ArrayIndexOutOfBoundsException: -97
Client> 	at org.lwjgl.input.Keyboard.getKeyName(Keyboard.java:429)
Client> 	at bas.a(SourceFile:624)
Client> 	at fa.a(SourceFile:45)
Client> 	at cj.b(SourceFile:64)
Client> 	at bas.d(SourceFile:92)
Client> 	at bbf.c(SourceFile:31)
Client> 	at asm.j(SourceFile:1091)
Client> 	... 3 more
Client> b@190a0d51
Client> 2013-04-20 01:14:13 [CLIENT] [INFO] Stopping!
Client> 2013-04-20 01:14:13 [SERVER] [INFO] Saving chunks for level '[13w16a] ADVENTURE!!!'/Nether
Client> 2013-04-20 01:14:13 [SERVER] [INFO] Saving chunks for level '[13w16a] ADVENTURE!!!'/The End
Client> 
Client> SoundSystem shutting down...
Client> 2013-04-20 01:14:14 [SERVER] [INFO] Stopping server
Client> 2013-04-20 01:14:14 [SERVER] [INFO] Saving players
Client> removing player mount
Client> 2013-04-20 01:14:14 [SERVER] [INFO] Saving worlds
Client> 2013-04-20 01:14:14 [SERVER] [INFO] Saving chunks for level '[13w16a] ADVENTURE!!!'/Overworld
Client> 2013-04-20 01:14:14 [SERVER] [INFO] Saving chunks for level '[13w16a] ADVENTURE!!!'/Nether
Client> 2013-04-20 01:14:14 [SERVER] [INFO] Saving chunks for level '[13w16a] ADVENTURE!!!'/The End

Steps to reproduce:
1. Bind "sneak" to "button 2" (right-click).
2. Bind "use item" to anything else ("h" for example)
3. Place a boat with "h".
4. "h" to enter boat - crash