media: mediatek: vcodec: Return NULL if no vdec_fb is found

"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: f77e8985 ("[media] vcodec: mediatek: Add Mediatek VP9 Video Decoder Driver")
Signed-off-by: Irui Wang <irui.wang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>