[BlobStorage] Fixing potential overflow

Bug: 779314
Change-Id: I74612639d20544e4c12230569c7b88fbe669ec03
Reviewed-on: https://chromium-review.googlesource.com/747725
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512977}