pop3.c: Fixed drop of final CRLF in EOB checking

Curl_pop3_write() would drop the final CRLF of a message as it was
considered part of the EOB as opposed to part of the message. Whilst
the EOB sequence needs to be searched for by the function only the
final 3 characters should be removed as per RFC-1939 section 3.

Reported by: Rich Gray
Bug: http://curl.haxx.se/mail/lib-2012-02/0051.html