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
smtp: Fixed inappropriate free of the scratch buffer

If the scratch buffer was allocated in a previous call to
Curl_smtp_escape_eob(), a new buffer not allocated in the subsequent
call and no action taken by that call, then an attempt would be made to
try and free the buffer which, by now, would be part of the data->state
structure.

This bug was introduced in commit 4bd860a001.