The output of `sfdisk --json /dev/sdb` is not necessarily
valid JSON. Then, no partition information is stored,
no first-valid-lba in particular. This leads to new partitions
being made from sector 0, which is invalid on a GPT table.
The workaround is to manually fix the known-broken JSON
from sfdisk. This is amply documented in a standalone
static function.
FIXES#425097
6aa9d93728