output/Control: move code to method StopThread()

src/output/Control.cxx

@@ -38,10 +38,7 @@ AudioOutputControl::AudioOutputControl(std::unique_ptr<FilteredAudioOutput> _out
 
 AudioOutputControl::~AudioOutputControl() noexcept
 {
-	BeginDestroy();
-
-	if (thread.IsDefined())
-		thread.Join();
+	StopThread();
 }
 
 void
@@ -383,3 +380,14 @@ AudioOutputControl::BeginDestroy() noexcept
 		}
 	}
 }
+
+void
+AudioOutputControl::StopThread() noexcept
+{
+	BeginDestroy();
+
+	if (thread.IsDefined())
+		thread.Join();
+
+	assert(IsCommandFinished());
+}

src/output/Control.hxx

@@ -566,6 +566,8 @@ private:
 	 */
 	void InternalDrain() noexcept;
 
+	void StopThread() noexcept;
+
 	/**
 	 * The OutputThread.
 	 */