libui: add boundary check to GraphicBuffer::unflatten

Commit cb496acbe593326e8d5d563847067d02b2df40ec removed the boundary
check by accident.

Bug: 114223584
Test: manual
Change-Id: I057bc02d5807e438530d1a5327c2e02b9d154151
(cherry picked from commit bf8d7210c4bbbdc875e9695a301cdf9c3b544279)
1 file changed