auto-compile--tree-member: Handle non-list root element

A top-level mode line construct may itself be something
other than a list (most likely a string).

Closes #31.
Closes #32.

auto-compile.el

@@ -270,12 +270,12 @@ non-nil."
 
 (defun auto-compile--tree-member (elt tree)
   ;; Also known as keycast--tree-member.
-  (or (member elt tree)
-      (catch 'found
-        (dolist (sub tree)
-          (when-let ((found (and (listp sub)
-                                 (auto-compile--tree-member elt sub))))
-            (throw 'found found))))))
+  (and (listp tree)
+       (or (member elt tree)
+           (catch 'found
+             (dolist (sub tree)
+               (when-let ((found (auto-compile--tree-member elt sub)))
+                 (throw 'found found)))))))
 
 (defun auto-compile-modify-mode-line (after)
   (let ((format (default-value 'mode-line-format)))