org-map-entries: Fix org-agenda-prepare-buffers call

* lisp/org.el (org-map-entries): Check that buffer-file-name is non-nil
  before passing to org-agenda-prepare-buffers.

Like 11e8749, this surfaced as an error when 05efa7a ("Backport commit
3a5f751 from Emacs master branch", 2015-08-08) replaced a
'(while (setq VAR (pop LIST)) BODY)' with '(dolist (VAR LIST) BODY)',
resulting in the body being executed when '(nil)' was passed to
org-agenda-prepare-buffers.

Reported-by: Robert Klein <roklein@roklein.de>
<http://permalink.gmane.org/gmane.emacs.orgmode/99779>

lisp/org.el

@@ -15490,7 +15490,7 @@ a *different* entry, you cannot use these techniques."
 	  (if (not scope)
 	      (progn
 		(org-agenda-prepare-buffers
-		 (list (buffer-file-name (current-buffer))))
+		 (and buffer-file-name (list buffer-file-name)))
 		(setq res (org-scan-tags func matcher todo-only start-level)))
 	    ;; Get the right scope
 	    (cond
@@ -15502,7 +15502,7 @@ a *different* entry, you cannot use these techniques."
 	      (setq scope (org-agenda-files t))
 	      (setq scope (org-add-archive-files scope)))
 	     ((eq scope 'file)
-	      (setq scope (list (buffer-file-name))))
+	      (setq scope (and buffer-file-name (list buffer-file-name))))
 	     ((eq scope 'file-with-archives)
 	      (setq scope (org-add-archive-files (list (buffer-file-name))))))
 	    (org-agenda-prepare-buffers scope)