Handle weirdly constructed PGP/MIME mails
* src/mapihelp.cpp (get_msgcls_from_pgp_lines): Add return value for empty body. (get_msgcls_from_first_attachment): New. factored out. (change_message_class_ipm_note): Use get_msgcls_from_first_attachment.
This fixes the handling of mails that are:
They also don't have a body. To avoid overhead in a
normal unencrypted multipart mixed mail we only check
for this structure if we don't have a body.
- GnuPG-Bug-Id: T3882