Crash while parsing command menu

[SamVanheer]

If a commandmenu.txt file is provided, and contains an entry for a custom button that isn't handled by the game, it will still try to access the button, resulting in a null pointer exception.

If this call returns null:

halflife/cl_dll/vgui_TeamFortressViewport.cpp

Line 836 in 5d76170

pButton = CreateCustomButton( cText, cCommand, iButtonY );

then this call:

halflife/cl_dll/vgui_TeamFortressViewport.cpp

Line 898 in 5d76170

m_pCommandMenus[m_iNumMenus] = CreateSubMenu(pButton, m_pCurrentCommandMenu, iButtonY );

Will end up accessing a null pointer.

Adding a check there prevents a crash, but will likely result in parser failure later on (using SDK command menu code with TFC's command menu causes parser failure).