Initialize DPB structures to valid values.

When the first frame is a B frame, the colocated picture
will now point to the current frame.

Test: run poc with and without this patch
Bug: 38115076
Change-Id: I48a8f128740551d6a9252931dafcf8c629ecad0d
(cherry picked from commit b8d362561e48dde8898eb0415f298d64e76f2b7c)