polarssl: security bump to version 1.2.13

Includes the previous CVE-2015-1182 fix (patch dropped) and other fixes
(security and non) from the 1.3 branch (no CVEs yet), see release notes:
https://polarssl.org/tech-updates/releases/polarssl-1.2.13-released



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>