assistant: fix a crash

Avoid the crash when handing stop interaction without an active assistant
manager. The root cause of this issue is still unknown.

Bug: 882952
Test: locally build and run
Change-Id: Ic7b9fc0f013ba926917a2b266235e887c169418b
Reviewed-on: https://chromium-review.googlesource.com/1240856Reviewed-by: Muyuan Li <muyuanli@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593677}

chromeos/services/assistant/assistant_manager_service_impl.cc

@@ -220,6 +220,11 @@ void AssistantManagerServiceImpl::StartVoiceInteraction() {
 void AssistantManagerServiceImpl::StopActiveInteraction(
     bool cancel_conversation) {
   platform_api_->SetMicState(false);
+
+  if (!assistant_manager_internal_) {
+    VLOG(1) << "Stopping interaction without assistant manager.";
+    return;
+  }
   assistant_manager_internal_->StopAssistantInteractionInternal(
       cancel_conversation);
 }