mbedtls: Fix session resume

This also fixes PolarSSL session resume.

Prior to this change the TLS session information wasn't properly
saved and restored for PolarSSL and mbedTLS.

Bug: https://curl.haxx.se/mail/lib-2016-01/0070.html
Reported-by: Thomas Glanzmann

Bug: https://curl.haxx.se/mail/lib-2016-04/0095.html
Reported-by: Moti Avrahami