spi25.c: Add a null check

Change-Id: I5fcc23d81b8404af90768afa2954509bf334ab2c
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/39974


Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>