Be more resilient against invalid headers in mml-generate-mime

* lisp/gnus/mml.el (mml-generate-mime): Don't bug out when called
with invalid headers (bug#55014).

1 files changed, 2 insertions, 1 deletions

diff --git a/lisp/gnus/mml.el b/lisp/gnus/mml.el
index 5a526025061..093e582ea7a 100644
--- a/lisp/gnus/mml.el
+++ b/lisp/gnus/mml.el
@@ -500,7 +500,8 @@ type detected."
       (when (and (consp (car cont))
                  (= (length cont) 1)
                  content-type)
-        (setcdr (assq 'type (cdr (car cont))) content-type))
+        (when-let ((spec (assq 'type (cdr (car cont)))))
+          (setcdr spec content-type)))
       (when (fboundp 'libxml-parse-html-region)
         (setq cont (mapcar #'mml-expand-all-html-into-multipart-related cont)))
       (prog1