mu4e-window: add mu4e--buffer-local-boundp

buffer-local-boundp was introduced in emacs 28.

Fixes #2456.

mu4e/mu4e-window.el

@@ -172,6 +172,17 @@ Checks are performed using `derived-mode-p' and the current
 buffer's major mode."
   (eq (mu4e--get-current-buffer-type) type))
 
+
+;; backward-compat; buffer-local-boundp was introduced in emacs 28.
+(defun mu4e--buffer-local-boundp (symbol buffer)
+  "Return non-nil if SYMBOL is bound in BUFFER.
+Also see `local-variable-p'."
+  (condition-case nil
+      (buffer-local-value symbol buffer)
+    (:success t)
+    (void-variable nil)))
+
+
 (defun mu4e-get-view-buffer (&optional headers-buffer create)
   "Return a view buffer belonging optionally to HEADERS-BUFFER.
 
@@ -197,8 +208,9 @@ being created if CREATE is non-nil."
           ;; Search all view buffers and return those that are linked to
           ;; `headers-buffer'.
           (linked-buffer (mu4e-get-view-buffers
-                          (lambda (buf) (and (buffer-local-boundp 'mu4e-linked-headers-buffer buf)
-                                        (eq mu4e-linked-headers-buffer headers-buffer))))))
+                          (lambda (buf)
+                            (and (mu4e--buffer-local-boundp 'mu4e-linked-headers-buffer buf)
+                                 (eq mu4e-linked-headers-buffer headers-buffer))))))
       ;; If such a linked buffer exists and its buffer is live, we use that buffer.
       (if (and linked-buffer (buffer-live-p (car linked-buffer)))
           ;; NOTE: It's possible for there to be more than one linked view buffer.