Hi all,

I came across an issue where Gnus would abort topic catchup when some
of the groups where from an unavailable server:

Debugger entered--Lisp error: (wrong-type-argument number-char-or-marker-p nil)
  gnus-sequence-of-unread-articles("nntp+news.optonline.net:comp.software.patterns")
  gnus-group-catchup("nntp+news.optonline.net:comp.software.patterns" nil)
  gnus-group-catchup-current()
  #<compiled-function (topic) "...(32)" [gnus-group-marked buffer-read-only gnus-level-killed topic groups call-interactively gnus-group-catchup-current mapcar #<compiled-function ... "...(5)" ... 2> gnus-topic-find-groups t nil gnus-topic-update-topics-containing-group] 8 ("/usr/lib/sxemacs/site-packages/lisp/gnus/gnus-topic.elc" . 25793) (list (gnus-group-topic-name))>("Engineering")
  call-interactively(gnus-topic-catchup-articles)

The following patch is a naive approach at solving it.
I realize I am just silencing the error, which might be bad, but it
works for me.