org-refile.el: Fix the case of emtpy buffer name

[satotake]

Dear Org maintainers,

I often use org-mode for note-taking and so on.
Sometimes, I would like to refile the headings in the scratch to other
org files while I am writing.
When I call `org-refile' in the buffer, Emacs throws errors on 
`org-refile-get-targets' because, typically,
these buffers do not have `buffer-name' and I set `org-refile-use-outline-path' 
`file'.
To fix it, I have created my first patch.

Regards
Takeshi SATO