efi_loader: check uuid_str_to_bin return value

Check the uuid_str_to_bin return value, skip the node
if the image-type-id property is wrong format.

Addresses-Coverity-ID: 463145 ("Error handling issues")
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>