trivial: Fix a tiny resource leak if the signature file cannot be opened
trivial: Fix a tiny resource leak if the signature file cannot be opened

Found using Coverity.

vathpela/fwupd,superm1/fwupd,vathpela/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,vathpela/fwupd,superm1/fwupd,hughsie/fwupd,vathpela/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,superm1/fwupd,fwupd/fwupd