Fix moving null fence reference

This CL fixes an earlier error where we might be pushing nullptr
reference into a vector.

Bug: 1144967, 1127984
Change-Id: Id80dc38b7ba7611661c080fa404e2fa7c69a762a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517601Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Khushal <khushalsagar@chromium.org>
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Auto-Submit: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823601}

gpu/command_buffer/service/shared_image_backing_gl_image.cc

@@ -252,7 +252,9 @@ bool SharedImageRepresentationOverlayImpl::BeginReadAccess(
     std::vector<gfx::GpuFence>* acquire_fences,
     std::vector<gfx::GpuFence>* release_fences) {
   auto* gl_backing = static_cast<SharedImageBackingGLImage*>(backing());
-  acquire_fences->push_back(std::move(*gl_backing->GetLastWriteGpuFence()));
+  std::unique_ptr<gfx::GpuFence> fence = gl_backing->GetLastWriteGpuFence();
+  if (fence)
+    acquire_fences->push_back(std::move(*fence));
   return true;
 }