(unicode-data): Check that

`unicodedata-file' exists.

lisp/international/mule-diag.el

@@ -1228,7 +1228,9 @@ looked up from it."
   "Return a list of Unicode data for unicode CHAR.
 Each element is a list of a property description and the property value.
 The list is null if CHAR isn't found in `unicodedata-file'."
-  (if unicodedata-file
+  (when unicodedata-file
+    (unless (file-exists-p unicodedata-file)
+      (error "`unicodedata-file' %s not found" unicodedata-file))
     (save-excursion
       (set-buffer (find-file-noselect unicodedata-file))
       (goto-char (point-min))