Traditional PGP checking isn't robust enough

[pdmef]

When viewing messages with $pgp_auto_decode set or using check-traditional-pgp, messages like http://marc.info/?l=mutt-users&m=121916523318263&w=2 look completely broken in mutt:

-----BEGIN PGP SIGNATURE-----
[-- BEGIN PGP SIGNED MESSAGE --]



[-- END PGP SIGNED MESSAGE --]

...which may be quite misleading.

Sure, it's and an edge-case with plain text that has parts looking like PGP, but displaying an empty message is wrong in this case.