Fix a bug where TestAudioDeviceModule crashes if destroyed uninitialized.

Because thread_ object is created in Init, destructor used to crash when
calling thread_->Stop() because it was referencing a null pointer.

Bug: webrtc:9404
Change-Id: I1c943d0fa50f9341aaa516b32495bb25bf4d664b
Reviewed-on: https://webrtc-review.googlesource.com/84122
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23682}
diff --git a/modules/audio_device/include/test_audio_device.cc b/modules/audio_device/include/test_audio_device.cc
index 9d833b3..e24afda 100644
--- a/modules/audio_device/include/test_audio_device.cc
+++ b/modules/audio_device/include/test_audio_device.cc
@@ -81,7 +81,9 @@
   ~TestAudioDeviceModuleImpl() {
     StopPlayout();
     StopRecording();
-    thread_->Stop();
+    if (thread_) {
+      thread_->Stop();
+    }
   }
 
   int32_t Init() {