Correct negative values for the next pre-allocated plus address.

If preference values on disk become corrupted, the index of the next
pre-allocated plus address may be read as negative. This would lead to
failed bounds checks.

Since we should handle errors that can be triggered from faulty I/O
data gracefully, this CL adds handling for negative values.

Bug: 324559503
Change-Id: I79bc316c9a6b9663ee233bdb4cdea5da924a180f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5771453
Reviewed-by: Kirubel Aklilu <kaklilu@chromium.org>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1339063}