fix(cksum): check metadata of the path (#1951)

* fix: check metadata of the path

* chore: use existing path

src/uu/cksum/src/cksum.rs

@@ -140,7 +140,20 @@ fn cksum(fname: &str) -> io::Result<(u32, usize)> {
     let mut rd: Box<dyn Read> = match fname {
         "-" => Box::new(stdin()),
         _ => {
-            file = File::open(&Path::new(fname))?;
+            let path = &Path::new(fname);
+            if path.is_dir() {
+                return Err(std::io::Error::new(
+                    io::ErrorKind::InvalidInput,
+                    "Is a directory",
+                ));
+            };
+            if !path.metadata().is_ok() {
+                return Err(std::io::Error::new(
+                    io::ErrorKind::NotFound,
+                    "No such file or directory",
+                ));
+            };
+            file = File::open(&path)?;
             Box::new(BufReader::new(file))
         }
     };