Decoder: Initialize slice parameters before concealing error MBs

Also memset ps_dec_op structure to zero.
For error input, this ensures dimensions are initialized to zero

Bug: 28165661
Change-Id: I66eb2ddc5e02e74b7ff04da5f749443920f37141
(cherry picked from commit 3bcac1f499cddc1761041b55169e27e92d89b3e3)
3 files changed