output/Control: avoid sending KILL twice

src/output/Control.cxx

@@ -377,7 +377,9 @@ AudioOutputControl::BeginDestroy() noexcept
 {
 	if (thread.IsDefined()) {
 		const std::lock_guard<Mutex> protect(mutex);
-		if (IsCommandFinished())
+		if (!killed) {
+			killed = true;
 			CommandAsync(Command::KILL);
+		}
 	}
 }

src/output/Control.hxx

@@ -212,6 +212,15 @@ class AudioOutputControl {
 	 */
 	bool skip_delay;
 
+	/**
+	 * Has Command::KILL already been sent?  This field is only
+	 * defined if `thread` is defined.  It shall avoid sending the
+	 * command twice.
+	 *
+	 * Protected by #mutex.
+	 */
+	bool killed;
+
 public:
 	/**
 	 * This mutex protects #open, #fail_timer, #pipe.

src/output/Thread.cxx

@@ -526,6 +526,8 @@ AudioOutputControl::StartThread()
 {
 	assert(command == Command::NONE);
 
+	killed = false;
+
 	const ScopeUnlock unlock(mutex);
 	thread.Start();
 }