Fix crash in |AssistantSettingsImpl::SyncSpeakerIdEnrollmentStatus|

This crash could happen if the enrollment update comes after the
Assistant has been stopped.

Bug: 1147895
Change-Id: I56b1db6e859fc24c621c36fe984f31168365fcf5
Tests: Compiled
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533194Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Yue Li <updowndota@chromium.org>
Commit-Queue: Jeroen Dhollander <jeroendh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828950}

chrome/browser/ui/webui/settings/chromeos/google_assistant_handler.cc

@@ -88,7 +88,9 @@ void GoogleAssistantHandler::HandleSyncVoiceModelStatus(
     const base::ListValue* args) {
   CHECK_EQ(0U, args->GetSize());
 
-  assistant::AssistantSettings::Get()->SyncSpeakerIdEnrollmentStatus();
+  auto* settings = assistant::AssistantSettings::Get();
+  if (settings)
+    settings->SyncSpeakerIdEnrollmentStatus();
 }
 
 void GoogleAssistantHandler::HandleInitialized(const base::ListValue* args) {