osutil: proper error checking and reporting

mercurial/osutil.c

@@ -255,7 +255,8 b' static PyObject *listdir(PyObject *self,'
 
 #ifdef AT_SYMLINK_NOFOLLOW
 	dfd = open(path, O_RDONLY);
-	dir = fdopendir(dfd);
+	if (dfd != -1)
+		dir = fdopendir(dfd);
 #else
 	dir = opendir(path);
 	dfd = -1;