output/MultipleOutputs: add method IsOpen()

src/output/MultipleOutputs.cxx

@@ -172,7 +172,7 @@ MultipleOutputs::Update(bool force) noexcept
 {
 	bool ret = false;
 
-	if (!input_audio_format.IsDefined())
+	if (!IsOpen())
 		return false;
 
 	for (const auto &ao : outputs)

src/output/MultipleOutputs.hxx

@@ -156,6 +156,15 @@ public:
 
 private:
 	/**
+	 * Was Open() called successfully?
+	 *
+	 * This method may only be called from the player thread.
+	 */
+	bool IsOpen() const noexcept {
+		return input_audio_format.IsDefined();
+	}
+
+	/**
 	 * Wait until all (active) outputs have finished the current
 	 * command.
 	 */