libdfu: Verify the GUsbDevice is set before using it
libdfu: Verify the GUsbDevice is set before using it

Handling this is important in a library that's going to be (ab)used by people.

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