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
Change-Id: Id8b851733b088cce0d07493fbf76e7e24f9299ad
(cherry picked from commit 9809602ac32dcb7bceaa5bc34df5b7fb68aacd38)
1 file changed