Allow parsing of message with 0 byte body.

I've come across a few emails without a message body.

The incoming SMTP conversation look something like this:

    DATA
    Date:...
    To:...
    Subject:....
    
    .

This generated a stream ending with "...Subject:...\r\n"

With the suggested changed the message will be parsed.
However I am not too familiar with the code, for example this change could misbehave in a stream ending inside the headers. Still it would be preferred if those messages could be parsed as well.