dm: Prevent "demo hello" and "demo status" segfaults

dm: Prevent "demo hello" and "demo status" segfaults

Segfaults can occur when a mandatory argument is not provided to
"demo hello" and "demo status".  Eg:

   => demo hello
   Segmentation fault (core dumped)

Add a check to ensure all required arguments are provided.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Acked-by: Simon Glass <sjg@chromium.org>