fetch a window even if its index is larger than window-count

The index of a window in window-vector can be greater than the number of live
windows if its number was assigned by winum-assign-func.

also partly fixes #4

winum.el

@@ -314,8 +314,9 @@ PREFIX must be a key sequence, like the ones returned by `kbd'."
 ;;;###autoload
 (defun winum-get-window-by-number (n)
   "Return window numbered N if exists, nil otherwise."
-  (when (and (>= n 0) (< n (1+ winum--window-count)))
-        (aref (winum--get-window-vector) n)))
+  (let ((window-vector (winum--get-window-vector)))
+    (when (and (>= n 0) (< n (length window-vector)))
+      (aref window-vector n))))
 
 ;;;###autoload
 (defun winum-get-number-string (&optional window)