Auto merge of #23593 - georgeroman:fix_panic_on_opening_directory, r=Manishearth

Fix panic on opening a directory

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

Continued from https://github.com/servo/servo/pull/23548

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23593)
<!-- Reviewable:end -->

components/net/fetch/methods.rs

@@ -613,6 +613,14 @@ fn scheme_fetch(
             }
             if let Ok(file_path) = url.to_file_path() {
                 if let Ok(file) = File::open(file_path.clone()) {
+                    if let Ok(metadata) = file.metadata() {
+                        if metadata.is_dir() {
+                            return Response::network_error(NetworkError::Internal(
+                                "Opening a directory is not supported".into(),
+                            ));
+                        }
+                    }
+
                     // Get range bounds (if any) and try to seek to the requested offset.
                     // If seeking fails, bail out with a NetworkError.
                     let file_size = match file.metadata() {