efi/chainloader: Propagate errors from copy_file_path()
    
    Without any error propagated to the caller, make_file_path()
    would then try to advance the invalid device path node with
    GRUB_EFI_NEXT_DEVICE_PATH(), which would fail, returning a NULL
    pointer that would subsequently be dereferenced. Hence, propagate
    errors from copy_file_path().
    
    Signed-off-by: Chris Coulson <chris.coulson@canonical.com>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>