Fix dylan-opt on buffer that isn't visiting a file

In that case (buffer-file-name) returns nil.

dylan-opt.el

@@ -243,21 +243,22 @@ can be used to toggle the optimization highlighting on and off."
 
 (defun dylan-opt--default-file-name ()
   "Guess a default optimization file to match the current buffer."
-  (let* ((path (buffer-file-name))
-         (name (file-name-nondirectory path))
-         (stem (substring name 0 (string-match "\\.[^.]*$" name)))
-         (library dylan-buffer-library))
-    (expand-file-name
-     (concat (or (getenv "OPEN_DYLAN_USER_ROOT") "_build")
-             "/build/" library "/" stem ".el"))))
+  (and (buffer-file-name)
+       (let* ((path (buffer-file-name))
+              (name (file-name-nondirectory path))
+              (stem (substring name 0 (string-match "\\.[^.]*$" name)))
+              (library dylan-buffer-library))
+         (expand-file-name
+          (concat (or (getenv "OPEN_DYLAN_USER_ROOT") "_build")
+                  "/build/" library "/" stem ".el")))))
 
 ;;;###autoload
 (defun dylan-opt (opt-file)
   "Show Dylan optimization faces according to OPT-FILE.
 
 See the command `dylan-opt-mode', which this command enables."
   (interactive
-   (list (let ((default (dylan-opt--default-file-name)))
+   (list (let ((default (or (dylan-opt--default-file-name) "")))
            (read-file-name
             "Dylan optimization file: "
             (file-name-directory default) nil t