Check error code in rust_file_is_dir. Prevent comparison of uninitialized mem

src/rt/rust_builtin.cpp

@@ -446,7 +446,9 @@ rust_dirent_filename(rust_task *task, dirent* ent) {
 extern "C" CDECL int
 rust_file_is_dir(rust_task *task, rust_str *path) {
     struct stat buf;
-    stat((char*)path->data, &buf);
+    if (stat((char*)path->data, &buf)) {
+        return 0;
+    }
     return S_ISDIR(buf.st_mode);
 }
 

src/test/stdtest/fs.rs

@@ -14,3 +14,8 @@ fn test_connect() {
 #[test]
 fn test_list_dir_no_invalid_memory_access() { fs::list_dir(~"."); }
 
+#[test]
+fn file_is_dir() {
+    assert fs::file_is_dir(~".");
+    assert !fs::file_is_dir(~"test/stdtest/fs.rs");
+}