Abort header parsing on invalid headers

* src/rfc822parse.c (insert_header): Return -1 on invalid header.
* src/mapihelp.cpp (parse_header_data): Handle rfc822_insert abort.

This fixes a problem where GpgOL would look into the first
attachment for header data in case no PR_TRANSPORT_HEADER property
could be found and then tried to parse the full attachment.

RT #8917