Fix denial-of-service issue in Gnus
Patch from emacs-28 branch

commit ae9bfed50dbf5043c0b47f20473ef43d8aeebebd
Author: Eli Zaretskii <eliz@gnu.org>
Date:   Mon Dec 19 19:01:04 2022 +0200

    Fix storing email into nnmail by Gnus

--- a/lisp/gnus/nnml.el
+++ b/lisp/gnus/nnml.el
@@ -775,17 +775,22 @@
 	(nnml--encode-headers headers)
 	headers))))
 
+;; RFC2047-encode Subject and From, but leave invalid headers unencoded.
 (defun nnml--encode-headers (headers)
   (let ((subject (mail-header-subject headers))
 	(rfc2047-encoding-type 'mime))
     (unless (string-match "\\`[[:ascii:]]*\\'" subject)
-      (setf (mail-header-subject headers)
-	    (mail-encode-encoded-word-string subject t))))
+      (let ((encoded-subject
+             (ignore-errors (mail-encode-encoded-word-string subject t))))
+        (if encoded-subject
+            (setf (mail-header-subject headers) encoded-subject)))))
   (let ((from (mail-header-from headers))
 	(rfc2047-encoding-type 'address-mime))
     (unless (string-match "\\`[[:ascii:]]*\\'" from)
-      (setf (mail-header-from headers)
-	    (rfc2047-encode-string from t)))))
+      (let ((encoded-from
+             (ignore-errors (rfc2047-encode-string from t))))
+        (if encoded-from
+            (setf (mail-header-from headers) encoded-from))))))
 
 (defun nnml-get-nov-buffer (group &optional incrementalp)
   (let ((buffer (gnus-get-buffer-create