trivial: Fix a memory leak if dfu-tool returns with an error
trivial: Fix a memory leak if dfu-tool returns with an error

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