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(Imported from upstream's 3661bb4e7934668bd99ca777ea8b30eedfafa871.)

Fix bug where i2c_ASN1_INTEGER mishandles zero if it is marked as
negative.

Thanks to Huzaifa Sidhpurwala <huzaifas@redhat.com> and Hanno Böck
<hanno@hboeck.de> for reporting this issue.

BUG=590615

(cherry-picked from c4eec0c16b02c97a62a95b6a08656c3a9ddb6baa)

Bug: 28175332
Change-Id: I8959e8ae01510a5924862a3f353be23130eee554
Reviewed-on: https://boringssl-review.googlesource.com/7199

Reviewed-by: David Benjamin <davidben@google.com>