efi_loader: memory leak in efi_set_variable()

Do not leak native_name if out of memory.

This addresses CoverityScan CID 184095.

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>