bug#9964: 23.3; Rmail is no longer able to display a summary buffer with no messages

[Mark Lillibridge]

    This is easy reproduce; just start Rmail on an empty file and hit
'h'.  Likewise, C-M-t on a nonempty Rmail file with a pattern that
matches no messages.

    Among other things, this breaks workflow in some cases.  For
example, if you have an rmail-buffer and its summary open, the later
window with focus, and are searching for messages, if you accidentally
pick a pattern with no matches, your summary buffer goes away and you
are left in a non-rmail-buffer which prevents you from just trying a
different pattern.

It also breaks the effect of:

(defcustom rmail-display-summary nil
  "If non-nil, Rmail always displays the summary buffer."
  :group 'rmail-summary
  :type 'boolean)


    I tried commenting out the code that checks for new messages, but
other things broke so more than a four-line patch will be required.  :-(

- Mark