efi: properly terminate filepath with NULL in chainloader

efi: properly terminate filepath with NULL in chainloader

EFI File Path Media Device Path is defined as NULL terminated string;
but chainloader built file paths without final NULL. This caused error
with Secure Boot and Linux Foundation PreLoader on Acer with InsydeH20 BIOS.
Apparently firmware failed verification with EFI_INVALID_PARAMETER which is
considered fatal error by PreLoader.

Reported and tested by Giovanni Santini <itachi.sama.amaterasu@gmail.com>