pjsip: Include timer patch to prevent cancelling timer 0.

I noticed this while looking at another issue and brought
it up with Teluu. It was possible for an uninitialized timer
to be cancelled, resulting in the invalid timer id of 0
being placed into the timer heap causing issues.

This change is a backport from the pjproject repository
preventing this from happening.

Change-Id: I1ba318b1f153a6dd7458846396e2867282b428e7