Fix up some byte-compiler warnings.

Fix up some byte-compiler warnings.
* lisp/gnus/gnus.el (gnus-group-find-parameter, gnus-kill-save-kill-buffer):
* lisp/gnus/gnus-cite.el (gnus-article-highlight-citation, gnus-dissect-cited-text)
(gnus-article-fill-cited-article, gnus-article-hide-citation)
(gnus-article-hide-citation-in-followups, gnus-cite-toggle):
* lisp/gnus/gnus-group.el (gnus-group-set-mode-line, gnus-group-quit)
(gnus-group-set-info, gnus-add-mark): Use with-current-buffer.
(gnus-group-update-group): Use save-excursion and with-current-buffer.