output/Thread: fix assertion failure in CLOSE handler

Convert assertion to runtime check; this assertion could fail when the
output was closed due to an error before CLOSE arrived.

src/output/OutputThread.cxx

@@ -443,9 +443,8 @@ AudioOutput::Task()
 			break;
 
 		case Command::CLOSE:
-			assert(open);
-
-			Close(false);
+			if (open)
+				Close(false);
 			CommandFinished();
 			break;