In the function ExecuteCommand, p_message is given in argument to ExecuteHelp
and is then modified locally in ExecuteHelp:
  p_message = vlm_MessageNew( "help", vlm_NULL );
whereas it is used further in function ExecuteCommand.
This patch fixes this problem by passing the pointer address in argument to
ExecuteHelp instead of the pointer itself.

Signed-off-by: Pierre d'Herbemont <pdherbemont@videolan.org>