The relinker currently blindly calls get_hashes on partitions in the
new, upper half of the partition space, which can cause extra handoffs
to be created if you're rerunning the relink step for safety.
Even if the relinker were smarter, though, there's no reason that a
curious operator should end up creating empty handoffs just because
they're poking devices with REPLICATE requests
Drive-by: be a little more strict about the "suffixes" we're willing to
put in hashes.invalid and read out of hashes.pkl.
Change-Id: I9aace80088cd00d02c418fe4d782b662fb5c8bcf
ade3b28636