[Reviewer: Mike] Enhance UTs to detect #209, and fix it

This allocates and releases a pool specifically for the rdata, so that Valgrind reports an invalid read when we try to access the Privacy header in ISCTest. It also takes my PJSIP fix, which fixes that invalid read warning.

I need to bring pjsip up to master once you've merged https://github.com/Metaswitch/pjsip-upstream/pull/22.