smbios: Do not fall back on devicetree without valid mapping

smbios: Do not fall back on devicetree without valid mapping

The smbios_get_val_si() function may get called for a sysinfo
property for which there is no mapping to a devicetree property.
Avoid a NULL pointer dereference in this case by skipping the
read of the mapped property from the device tree.

Fixes: 83b28b55d74f ("smbios: add support for dynamic generation of Type 9 system slot tables")
Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Raymond Mao <raymondmaoca@gmail.com>