Fix bug in org-remember.el

Fix bug in org-remember.el

Adam Spiers writes:

> The code for handling %^{PROP}p in org-remember templates seems to
> have a bug:
>
> 	     ((equal char "p")
>              (edebug)
> 	      (let*
> 		  ((prop (org-substring-no-properties prompt))
> 		   (pall (concat prop "_ALL"))
> 		   (allowed
> 		    (with-current-buffer
> 			(get-buffer (file-name-nondirectory file))
>
> Here get-buffer can return nil, in which case `org-remember' fails
> with the error:
>
>  Wrong type argument: stringp, nil
>
> It seems to be assuming that the non-directory part of the filename
> corresponds to the buffer name, but this is not guaranteed.  Perhaps I
> noticed it because I am using uniquify.el ?