smtp: Fixed dot stuffing when EOL characters were at end of input buffers

Fixed a problem with the CRLF. detection when multiple buffers were
used to upload an email to libcurl and the line ending character(s)
appeared at the end of each buffer. This meant any lines which started
with . would not be escaped into .. and could be interpreted as the end
of transmission string instead.

This only affected libcurl based applications that used a read function
and wasn't reproducible with the curl command-line tool.

Bug: http://curl.haxx.se/bug/view.cgi?id=1456
Assisted-by: Patrick Monnerat