gcc-mark-as-read sometimes broken on Exchange

[Dave Goldberg <david.goldberg6@verizon.net>]

Since doing a pull to deal with my earlier issue (latest commit is ca8599c49933f974e07ce3ece5d4694bd1956b06) I've been noticing that periodically the code to mark gcc's as read is generating this backtrace on my Exchange server.

Debugger entered--Lisp error: (wrong-type-argument number-char-or-marker-p nil)
  gnus-add-to-range(((1 . 28007)) (nil))
  gnus-compute-read-articles("nnimap+exchange:Sent Items" (nil))
  gnus-group-make-articles-read("nnimap+exchange:Sent Items" (nil))
  gnus-group-mark-article-read("nnimap+exchange:Sent Items" nil)
  gnus-inews-do-gcc()
  gnus-agent-possibly-do-gcc()
  run-hooks(gnus-agent-possibly-do-gcc)
  message-send(nil)
  message-send-and-exit(nil)
  call-interactively(message-send-and-exit)

The gcc has indeed succeeded, and both gcc and marking as read has succeeded on my cyrus server (I gcc to both) and most of the time marking as read succeeds on Exchange too.  So repeating this is not easy.  I assume that the `nil' there is supposed to have the article number.  A quick look at the code doesn't really tell me why it might be calculated as nil, but I'm no lisp expert.  This could have been introduced any time in the last two to two and a half weeks as that's the previous time I updated.

Thanks,
-- 
Dave Goldberg
david.goldberg6@verizon.net