fmt: generate an error if the input is a directory

tested by tests/misc/read-errors

src/uu/fmt/src/fmt.rs

@@ -189,6 +189,13 @@ fn process_file(
         _ => {
             let f = File::open(file_name)
                 .map_err_context(|| format!("cannot open {} for reading", file_name.quote()))?;
+            if f.metadata()
+                .map_err_context(|| format!("cannot get metadata for {}", file_name.quote()))?
+                .is_dir()
+            {
+                return Err(USimpleError::new(1, "read error".to_string()));
+            }
+
             Box::new(f) as Box<dyn Read + 'static>
         }
     });

tests/by-util/test_fmt.rs

@@ -9,6 +9,11 @@ fn test_invalid_arg() {
     new_ucmd!().arg("--definitely-invalid").fails().code_is(1);
 }
 
+#[test]
+fn test_invalid_input() {
+    new_ucmd!().arg(".").fails().code_is(1);
+}
+
 #[test]
 fn test_fmt() {
     new_ucmd!()