Fix crash in AudioInputImpl

This crash was introduced in my recent refactoring.
Note it only happens when VLOG is enabled.

Bug: N/A
Test: deployed.
Change-Id: Ie7029a2cf250a93778a831af95596ce02cbf8809
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612087
Auto-Submit: Jeroen Dhollander <jeroendh@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840392}

chromeos/services/assistant/platform/audio_input_impl.cc

@@ -281,7 +281,8 @@ void AudioInputImpl::AddObserver(
 void AudioInputImpl::RemoveObserver(
     assistant_client::AudioInput::Observer* observer) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(observer_sequence_checker_);
-  VLOG(1) << open_audio_stream_->device_id() << " remove observer";
+  if (open_audio_stream_)
+    VLOG(1) << open_audio_stream_->device_id() << " remove observer";
   bool have_no_observer = false;
   {
     base::AutoLock lock(lock_);