standalone-installer: Fix issue found by LGTM
standalone-installer: Fix issue found by LGTM

See https://lgtm.com/rules/6780080/ for details.

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