Check returned volumes before accessing them
Check returned volumes before accessing them

Without udisks2 installed the list of volumes is empty and fwupdtool
esp-list segfaults.

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