Fix a bug where TestAudioDeviceModule crashes if destroyed uninitialized.
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 <eb0877843acc39f8ef6f7269937dee931c372d23@webrtc.org>
Reviewed-by: Artem Titov <1e27363ccc366a7ff0fdde48204d68d5ebfeccc5@webrtc.org>
Commit-Queue: Sami Kalliomäki <b9debeb8943b89663e00c95863751401cba91c9a@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#23682}
TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc