Discard stdout if not redirected to file

Commit 97bee661 Printf statement at widgets/msgviewer.go#188 introduced
bad formatting in the display if stdout was not being redirected.

aerc.go

@@ -128,6 +128,7 @@ func main() {
 		logOut = os.Stdout
 	} else {
 		logOut = ioutil.Discard
+		os.Stdout, _ = os.Open(os.DevNull)
 	}
 	logger = log.New(logOut, "", log.LstdFlags)
 	logger.Println("Starting up aerc")