bug#51733: 27.1; Detect impossible email addresses better

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: Lars Ingebrigtsen <larsi@gnus.org>,  51733@debbugs.gnu.org
> Date: Wed, 19 Jan 2022 10:21:22 +0100
> 
>     Eli> Does textsec-email-suspicious-p expect non-ASCII email addresses to 
> be
>     Eli> RFC 2047 encoded?  If so, it will not work in the Rmail display
>     Eli> buffers, where email addresses are shown decoded.  For non-ASCII 
> names
>     Eli> the function signals an error.
> 
> It does? Do you have an example? The following works fine

Here:

  (textsec-email-suspicious-p "אבגד <foo@bar.com>")
    => (wrong-type-argument stringp nil)

with this backtrace:

  Debugger entered--Lisp error: (wrong-type-argument stringp nil)
    string-search("=?" nil)
    rfc2047-decode-string(nil)
    mail-header-parse-address("אבגד <foo@bar.com>" t)
    textsec-email-suspicious-p("אבגד <foo@bar.com>")
    (progn (textsec-email-suspicious-p "אבגד <foo@bar.com>"))
    eval((progn (textsec-email-suspicious-p "אבגד <foo@bar.com>")) t)
    elisp--eval-last-sexp(t)
    eval-last-sexp(t)
    eval-print-last-sexp(nil)
    funcall-interactively(eval-print-last-sexp nil)
    call-interactively(eval-print-last-sexp nil nil)
    command-execute(eval-print-last-sexp)