chown: fails when XXXX. or XXXX: is provided (when XXXX is numeric values)

If the arg starts with an id numeric value, the group isn't set but the separator is provided,
we should fail with an error

Should fix tests/chown/separator.sh

tests/by-util/test_chown.rs

@@ -423,6 +423,38 @@ fn test_chown_only_user_id() {
         .stderr_contains("failed to change");
 }
 
+#[test]
+fn test_chown_fail_id() {
+    // test chown 1111. file.txt
+
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    let result = scene.cmd_keepenv("id").arg("-u").run();
+    if skipping_test_is_okay(&result, "id: cannot find name for group ID") {
+        return;
+    }
+    let user_id = String::from(result.stdout_str().trim());
+    assert!(!user_id.is_empty());
+
+    let file1 = "test_chown_file1";
+    at.touch(file1);
+
+    scene
+        .ucmd()
+        .arg(format!("{}:", user_id))
+        .arg(file1)
+        .fails()
+        .stderr_contains("invalid spec");
+
+    scene
+        .ucmd()
+        .arg(format!("{}.", user_id))
+        .arg(file1)
+        .fails()
+        .stderr_contains("invalid spec");
+}
+
 /// Test for setting the owner to a user ID for a user that does not exist.
 ///
 /// For example: