WIP: Should use rhost?

WIP: Should use rhost?

I'm not sure this is the correct fix but the current code tries to
duplicate the string 'relayhost' which is always NULL at this point.

   In file included from send_mail.c:39:
   In function ‘xstrdup’,
       inlined from ‘newsmtp’ at send_mail.c:362:15:
   ../include/xmalloc.h:34:19: warning: argument 1 null where non-null expected [-Wnonnull]
      34 |         char *s = strdup(str);
         |                   ^~~~~~~~~~~
   In file included from ../include/log_error.h:28,
                    from send_mail.c:34:
   /usr/include/string.h: In function ‘newsmtp’:
   /usr/include/string.h:187:14: note: in a call to function ‘strdup’ declared ‘nonnull’
     187 | extern char *strdup (const char *__s)
         |              ^~~~~~

Signed-off-by: Michael Jeanson <mjeanson@debian.org>