Instead return just NULL. This matches the behaviour of kstrdup in the
kernel and xstrdup in busybox.
This fixes a panic with CONFIG_CMD_MAGICVAR=y and
CONFIG_CMD_MAGICVAR_HELP unset in magicvar_add() where description is
always NULL.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>