Fix crash while parsing command menu with unsupported custom button.

Same as 3e0214ee12

cl_dll/vgui_TeamFortressViewport.cpp

@@ -841,7 +841,7 @@ int TeamFortressViewport::CreateCommandMenu( const char *menuFile, int direction
 					{
 						gEngfuncs.Con_Printf( "Too many menus in %s past '%s'\n", menuFile, szLastButtonText );
 					}
-					else
+					else if( pButton )
 					{
 						// Create the menu
 						m_pCommandMenus[m_iNumMenus] = CreateSubMenu( pButton, m_pCurrentCommandMenu, iButtonY );