Fix decoding of non-ASCII email attachments

* lisp/mail/rfc2231.el (rfc2231-parse-string): Fix logic when a
non-ASCII file name is split between two filename*N* parts.
(Bug#74624)

lisp/mail/rfc2231.el

@@ -193,7 +193,7 @@ must never cause a Lisp error."
 		     (push (list attribute value encoded) cparams))
 		    ;; Repetition of a part; do nothing.
 		    ((and elem
-			  (null number))
+			  (null part))
 		     )
 		    ;; Concatenate continuation parts.
 		    (t