spi: Fix a oss-fuzz detected critical warning
spi: Fix a oss-fuzz detected critical warning

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32249

fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd