org-make-tags-matcher: Do not try collecting tags from non-Org buffers

* lisp/org.el (org-make-tags-matcher): When building tag completion
table, do not attempt to call `org-get-buffer-tags' when current
buffer is not an Org buffer.

Reported-by: Colin Baxter <m43cap@yandex.com>
Link: https://orgmode.org/list/87ilb1fakk.fsf@yandex.com

lisp/org.el

@@ -11288,7 +11288,8 @@ See also `org-scan-tags'."
     ;; tags table and the local tags in current buffer.
     (let ((org-last-tags-completion-table
 	   (org--tag-add-to-alist
-	    (org-get-buffer-tags)
+            (when (derived-mode-p 'org-mode)
+	      (org-get-buffer-tags))
 	    (unless only-local-tags
 	      (org-global-tags-completion-table)))))
       (setq match