smtp: Fixed an issue with the multi-interface always sending postdata

Due to the result code being reset to CURLE_OK when smtp_dophase_done()
was called, postdata would incorrectly be sent to the server when the
MAIL FROM or RCPT command was rejected.

As such, libcurl would return the wrong result code from performing the
operation and additionally set CURLINFO_RESPONSE_CODE to be that
returned by the postdata command.

Bug: http://curl.haxx.se/mail/lib-2012-05/0108.html
Reported by: Gokhan Sengun