[Dovecot] message parser: Fixed infinite loop when parsing a specific message.

[Timo Sirainen]

I committed this change to all hg branches:
http://hg.dovecot.org/dovecot-2.1/rev/4461b48fcc1f

After that I realized that it doesn't actually matter, because it fixes
only a situation where input buffer's size is less than 84 bytes. This
happened on a test program where I was using a 64 byte buffer, but the
real code in Dovecot always uses much larger buffers.

So, don't worry, there's no way to actually DOS Dovecot with this. No
need for distro people to create any security releases.