smtp: Fixed intermittent "SSL3_WRITE_PENDING: bad write retry" error

This patch fixes the "SSL3_WRITE_PENDING: bad write retry" error that
sometimes occurs when sending an email over SMTPS with OpenSSL. OpenSSL
appears to require the same pointer on a write that follows a retry
(CURLE_AGAIN) as discussed here:

http://stackoverflow.com/questions/2997218/why-am-i-getting-error1409f07fssl-routinesssl3-write-pending-bad-write-retr