Fixes crash on Linux that surfaced as a result of r12493. But, this prompted

research into why that code had been ifdef'd in the first place and allowed
the root cause to be identified.

Now the menus are cleaned up properly, which also fixes bug #458 without
having the workaround in r12639.

Basically, the problem was that attempts were being made to manipulate
menuitems on a menu that no longer existed (after the menus were rebuilt).

src/commands/CommandManager.cpp

@@ -165,6 +165,7 @@ void CommandManager::PurgeData()
    mCurrentMenu = NULL;
    mCurrentMenuName = COMMAND;
    mCurrentID = 0;
+   mOpenMenu = NULL;
 }