Package: gnus

The fix for Gnus bug #19573 [1] seems to have introduced a problem for me when
exiting summary buffers: the `gnus-newsgroup-name' variable can become
set to the empty string buffer-locally in the summary buffer, and when
that happens Gnus errors with "Empty group name" when hitting 'q'.

This happens if I invoke `gnus-msg-mail' from a summary buffer, and
either just kill or send the message, returning me back to the summary.

Looking at code, it seems like `gnus-newsgroup-name' is restored from
let-bound `group-name' in an unwind form after `gnus-setup-message', but
the problem is it becomes set buffer locally in the newly opened
*message* buffer, and not what I believe was the intention: the original
summary buffer ?

[1] http://git.savannah.gnu.org/cgit/emacs.git/commit/lisp/gnus/gnus-msg.el?h=emacs-25.1-rc2&id=f61c87f12a36bb2063c25b6742380b5916618ab5

Currently, I am testing the following patch which seems to work, but I
cannot be sure about it, because I do not fully understand the purpose
of setting `gnus-newsgroup-name' to the empty string in the first place.