ox-icalendar: Fix BBDB anniversaries integration

* lisp/ox-icalendar.el (org-icalendar--combine-files): Make sure
  anniversaries do not end up in *Message* buffer instead of the ICS
  file.

Patch-by: Thomas Baumann <tbaumann@tum.de>

lisp/ox-icalendar.el

@@ -964,9 +964,7 @@ files to build the calendar from."
 	     ;; BBDB anniversaries.
 	     (when (and org-icalendar-include-bbdb-anniversaries
 			(require 'org-bbdb nil t))
-	       (with-temp-buffer
-		 (org-bbdb-anniv-export-ical)
-		 (buffer-string)))))))
+	       (with-output-to-string (org-bbdb-anniv-export-ical)))))))
 	(run-hook-with-args 'org-icalendar-after-save-hook
 			    org-icalendar-combined-agenda-file))
     (org-release-buffers org-agenda-new-buffers)))