u2v video out filename should be mandatory

main.go

@@ -189,6 +189,9 @@ func main() {
 		downloadUgoiraCmd(downloadCmd.Args(), flags)
 	case "u2v":
 		u2vCmd.Parse(os.Args[2:])
+		if len(flags.outputFileName) == 0 {
+			die("Expected filename output for video")
+		}
 		ugoira2videoCmd(u2vCmd.Args(), flags)
 	case "-h":
 		fmt.Printf("Commands: download, u2v\nUse '%s <command> -h' to see the command-specific flags.\n", os.Args[0])