Fix crash when we don't have a plane to display

ui/ozone/platform/dri/overlay_plane.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/ozone/platform/dri/overlay_plane.h"
 
 #include "base/logging.h"
 #include "ui/ozone/platform/dri/scanout_buffer.h"
 
 namespace ui {
@@ skipping 14 matching lines @@
       z_order(z_order),
       plane_transform(plane_transform),
       display_bounds(display_bounds),
       crop_rect(crop_rect) {
 }
 
 OverlayPlane::~OverlayPlane() {
 }
 
 // static
-const OverlayPlane& OverlayPlane::GetPrimaryPlane(
+const OverlayPlane* OverlayPlane::GetPrimaryPlane(
     const OverlayPlaneList& overlays) {
   for (size_t i = 0; i < overlays.size(); ++i) {
     if (overlays[i].z_order == 0)
-      return overlays[i];
+      return &overlays[i];
   }
 
-  NOTREACHED();
-  return overlays[0];
+  return nullptr;
 }
 
 }  // namespace ui