org-bibtex-yank: Fix bibtex parser not initialized in temp buffer

* lisp/ol-bibtex.el (org-bibtex-yank): Make sure that we parse bibtex
entry from the kill ring in a `bibtex-mode' buffer.  Otherwise,
calling `org-bibtex-read' (that calls `bibtex-parse-entry') may err
because some Bibtex parser variables are not initialized.

Reported-by: J. G. <jerzor@pacbell.net>
Link: https://orgmode.org/list/1939460027.3272000.1691771671040@mail.yahoo.com

lisp/ol-bibtex.el

@@ -761,7 +761,10 @@ drawer."
   "If kill ring holds a bibtex entry yank it as an Org headline."
   (interactive)
   (let (entry)
-    (with-temp-buffer (yank 1) (setf entry (org-bibtex-read)))
+    (with-temp-buffer
+      (yank 1)
+      (bibtex-mode)
+      (setf entry (org-bibtex-read)))
     (if entry
 	(org-bibtex-write)
       (error "Yanked text does not appear to contain a BibTeX entry"))))