Restore body on verification error

* src/mail.cpp (Mail::update_body): Restore body if verify
fails.
(Mail::decrypt_verify): Save original body.

If there is an error during verification we should not
"eat" the body. Mostly this will be a technical error
and as we show the result as "insecure" this should
be ok.

• GnuPG-Bug-Id: T3538