Fix other runtime issues in XEmacs and Emacs 21.

* tex-buf.el (TeX-next-error): Do not pass `reparse' argument to
`next-error' in XEmacs as it is not supported.
(TeX-error-overview-frame, TeX-error-overview-buffer-name): Move
before their first use in order to prevent a runtime error in
XEmacs and GNU Emacs 21.  Reported by Ikumi Keita.
(TeX-parse-TeX): Manually set `item' to nil when
`TeX-error-last-visited' is negative.
(TeX-error-description-error, TeX-error-description-warning)
(TeX-error-description-tex-said): Set to nil in XEmacs and GNU
Emacs 21.  Reported by Ikumi Keita.