Initializing reference list for every P/B slice.

Reference list needs to be initialized for every P/B
slice, to ensure colocated picture always points to a
valid picture buffer, even in the case of error.

Bug: 36279112

Change-Id: I051d7e725b0af209cc7bb333db8da3518adf78a0
(cherry picked from commit f9d3f9af8fc113acda28e1a4e48d85736ee29c75)