Introduced by #9765
When a backup merge fails on a VHD file, the footer might be missing. This caused a footer1 !== footer2 error and triggered a full backup while not deleting the previous chain and resulted in all further backup jobs to be displayed as failures even though the new backup chain was working properly.
The fix allows the cleanVm to resume the merge where it stops even if the VHD file footer is missing.
63f8d624fd