Fix critical warning when parsing invalid FDT firmware
Fix critical warning when parsing invalid FDT firmware

Never let firmware_current be NULL by checking for the root node ENDing.

Fixes https://oss-fuzz.com/testcase-detail/5454411320655872

fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd