mm(1): fixes mm without arguments not working

src/mm.rs

@@ -131,6 +131,11 @@ fn main() -> ExitCode {
 		}
 	}
 
+	if ins.is_empty() && outs.is_empty() && argv.len() > optind {
+		eprintln!("Usage: {}", usage);
+		return ExitCode::from(EX_USAGE as u8);
+	}
+
 	/* use stdin if no inputs are specified */
 	if ins.is_empty() { ins.push("-".to_string()); }