fop(1): fixes panic on no arguments

src/fop.rs

@@ -61,6 +61,8 @@ fn main() -> ExitCode {
 		}
 	}
 
+	if argv.len() == 1 { return ExitCode::from(usage(&argv[0])); }
+
 	while let Some(opt) = argv.getopt("d:") {
 		match opt.opt() {
 			Ok("d") => {