ui: Fix bad size check in Fence::unflatten

Differs slightly from mnc+ patch: GetFlattenedSize was fixed in mnc.

Test: Boot device, run poc from bug, observe no longer crashes
Bug: 37285689
AOSP-Change-Id: Id8b851733b088cce0d07493fbf76e7e24f9299ad
(cherry picked from commit 9809602a)

CVE-2017-0666

Change-Id: I778c82b363ca0409d534f255cc5d17b39e751986