SMFIC_MAIL: bad packet when Postfix dispatches a DSN via non_smtpd_milters

[irsl]

From http://www.postfix.org/MILTER_README.html#non-smtp-milters:
"There's one small complication when using Milter applications for non-SMTP mail: there is no SMTP session. To keep Milter applications happy, the Postfix cleanup(8) server actually has to simulate the SMTP client CONNECT and DISCONNECT events, and the SMTP client EHLO, MAIL FROM, RCPT TO and DATA commands. "

This is actually reflected by dispatching an envfrom callback with an empty payload,which makes the lib die with "SMFIC_MAIL: bad packet" error message (Context.pm, line 267). Turning it into a warn fixes the problem and the filter then works as expected.