output/Control: add LockWaitForCommand()

src/output/Control.hxx

@@ -309,6 +309,11 @@ public:
 	 */
 	void WaitForCommand() noexcept;
 
+	void LockWaitForCommand() noexcept {
+		const std::lock_guard<Mutex> protect(mutex);
+		WaitForCommand();
+	}
+
 	/**
 	 * Sends a command, but does not wait for completion.
 	 *

src/output/MultipleOutputs.cxx

@@ -158,10 +158,8 @@ MultipleOutputs::EnableDisable()
 void
 MultipleOutputs::WaitAll() noexcept
 {
-	for (const auto &ao : outputs) {
-		const std::lock_guard<Mutex> protect(ao->mutex);
-		ao->WaitForCommand();
-	}
+	for (const auto &ao : outputs)
+		ao->LockWaitForCommand();
 }
 
 void