avoid error when message-fetch-field returns nil

* gnus-art.el (article-hide-boring-headers): Use "" when
  `message-fetch-field' returns `nil'

diff --git a/lisp/gnus/gnus-art.el b/lisp/gnus/gnus-art.el
index e28d84e06fe2b291e3e99dec0840a353fcf1fcf7..18baf982b2bd2992514e73fb3b6b98ec0b503ac6 100644 (file)
--- a/lisp/gnus/gnus-art.el
+++ b/lisp/gnus/gnus-art.el
@@ -1987,8 +1987,8 @@ always hide."
                  (gnus-article-hide-header "cc"))))
             ((eq elem 'followup-to)
              (when (string-equal-ignore-case
-                    (message-fetch-field "followup-to")
-                    (message-fetch-field "newsgroups"))
+                    (or (message-fetch-field "followup-to") "")
+                    (or (message-fetch-field "newsgroups") ""))
                (gnus-article-hide-header "followup-to")))
             ((eq elem 'reply-to)
              (if (gnus-group-find-parameter