"fb_use_list" is used to store used or referenced frame buffers for
vp9 stateful decoder. "NULL" should be returned when getting target
frame buffer failed from "fb_use_list", not a random unexpected one.
Fixes: f77e89854b3e ("[media] vcodec: mediatek: Add Mediatek VP9 Video Decoder Driver")
Signed-off-by: Irui Wang <irui.wang@xxxxxxxxxxxx>