efi_loader: variable: check a return value of uuid__str_to_bin()

The only error case is that a given UUID is in wrong format.
So just return EFI_INVALID_PARAMETER here.

Reported-by: Coverity (CID 300333)
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>