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>

common/cmd_demo.c

@@ -97,7 +97,9 @@ static int do_demo(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 				ARRAY_SIZE(demo_commands));
 	argc -= 2;
 	argv += 2;
-	if (!demo_cmd || argc > demo_cmd->maxargs)
+
+	if ((!demo_cmd || argc > demo_cmd->maxargs) ||
+	    ((demo_cmd->name[0] != 'l') && (argc < 1)))
 		return CMD_RET_USAGE;
 
 	if (argc) {