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>

drivers/media/platform/mediatek/vcodec/vdec/vdec_vp9_if.c

@@ -226,10 +226,11 @@ static struct vdec_fb *vp9_rm_from_fb_use_list(struct vdec_vp9_inst
 		if (fb->base_y.va == addr) {
 			list_move_tail(&node->list,
 				       &inst->available_fb_node_list);
-			break;
+			return fb;
 		}
 	}
-	return fb;
+
+	return NULL;
 }
 
 static void vp9_add_to_fb_free_list(struct vdec_vp9_inst *inst,