Fix handling of inline PGP Mails with attachment

* src/mapihelp.cpp (mapi_mark_or_create_moss_attach): Repeat check
after attachment creation.

The created MOSS attachment does not neccessary has position 1
it might change depending on the attachment count so the wrong
attachment may be picked up. Repeat the check for our newly
created attachment in that case.

• GnuPG-Bug-Id: T3365