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This change fixes a bug in U2fSign, where under certain circumstances an
end iterator was dereferenced.

Bug: 833398
Change-Id: I9194a966b01fbe9da6e51e50645f7f301e0d59e5
Reviewed-on: https://chromium-review.googlesource.com/1013484
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551299}