libutils: fix overflow in SharedBuffer [DO NOT MERGE]

See
https://code.google.com/p/android/issues/detail?id=181910

Bug: 22952485
(cherry picked from commit 66b6eb9490beeeabc804d790c1c4060ce047afd4)

Change-Id: Ic71dd0025b9a7588c4f3bb1c7be1bd13d2ff5105
3 files changed