Don't disconnect a player if they're already disconnected (fixes BUKKIT-43)

src/main/java/net/minecraft/server/ServerConfigurationManager.java

@@ -152,6 +152,8 @@ public class ServerConfigurationManager {
     }
 
     public String disconnect(EntityPlayer entityplayer) { // CraftBukkit - changed return type
+        if (entityplayer.netServerHandler.disconnected) return null; // CraftBukkit - exploitsies fix
+
         // CraftBukkit start
         // Quitting must be before we do final save of data, in case plugins need to modify it
         this.getPlayerManager(entityplayer.dimension).removePlayer(entityplayer);