Fix prev slice incomplete check

Bug: 64552185
Bug: 65034175

Move the decision to signal missing ctbs as skipped ctbs of
previous slice to the end of current slice hdr parsing to
ensure no error in slice hdr parsing.

Change-Id: Ia33b4be31dad6225b04e7b20b9000059c87941c4
(cherry picked from commit 74f35f191022c06863dacd33a3651a33be3ef08d)
(cherry picked from commit 9c24714e25d6ba713549a573df1d496479dce2b1)
