Fix bug in mapping entries

Fix bug in mapping entries

Patch by Peter Jones, following a bug report by Xiao-Jong Jin, who wrote:

> If you have the follow org file
>
> * test crypt							      :crypt:
> ** subheading 1
>   text 1
> ** subheading 2
>   text 2
>
> with setup as
>
> (require 'org-crypt)
> (setq org-tags-exclude-from-inheritance '("crypt"))
> (setq org-crypt-key "CBC0714E")              ; my key
>
> On calling org-encrypt-entry on the first head line, only
> subheading 1 get encrypted, subheading 2 remains plain text.
> But, if you add an empty line or some text under the first
> heading, both subheading 1 and 2 are encrypted.