libdfu: Fix verification of written DfuSe firmware
libdfu: Fix verification of written DfuSe firmware

DfuSe doesn't stop returning data when the image is empty. Take as least as
much data as we require and then truncate the result to what we want.

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