Handle detected attachments without filenames

* src/mail.cpp (Mail::add_attachments): Ingore attachments without
filenames.
(Mail::parsing_done): Don't fail hard on attachment add failure.

This fixes the case if some mimestructure or unknown multipart/mixed
attachments like the "private-headers" mimepart from Enigmail is
present. Before this the first empty attachment filename would
result in an error and no other attachments would have been shown.