common: bootm: check return value of strict_strtoul

Before continue, check return value of strict_strtoul.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Simon Glass <sjg@chromium.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: York Sun <yorksun@freescale.com>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>