efi: efibc: Guard against allocation failure

There is a single kmalloc in this driver, and it's not currently
guarded against allocation failure. Do it here by just bailing-out
the reboot handler, in case this tentative allocation fails.

Fixes: 416581e4

 ("efi: efibc: avoid efivar API for setting variables")
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>