cros: Fix crash after menu closes

After the menu closes, rendered content can be delivered
using a null |deliver_image_model_callback_|, so check for it
before calling.

Bug: 1134418
Change-Id: Ic97f3871a201a1c289835df74b891edf9d616460
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491141Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821297}

chrome/browser/ui/ash/clipboard_image_model_request.cc

@@ -180,6 +180,11 @@ void ClipboardImageModelRequest::CopySurface() {
 }
 
 void ClipboardImageModelRequest::OnCopyComplete(const SkBitmap& bitmap) {
+  if (!deliver_image_model_callback_) {
+    Stop();
+    return;
+  }
+
   std::move(deliver_image_model_callback_)
       .Run(ui::ImageModel::FromImageSkia(
           gfx::ImageSkia::CreateFrom1xBitmap(bitmap)));