trivial: Fix a potential division by zero
trivial: Fix a potential division by zero

Spotted by Coverity.

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