vp9-svc: Fix to avoid msan unitialized value.

Move the logic for forcing zero_mode after the
(ref_frame & flag_list) check.
This was causing an memory leak under msan:
https://bugs.chromium.org/p/webrtc/issues/detail?id=5402

Change-Id: Ie9d243369f8ed7c332f46178275945331da4fd85