res_pjsip, res_pjsip_session: initialize local variables

This patch initializes a couple of local variables to some default values.
Interestingly, in the 'pj_status_t dlg_status' case the value not being
initialized caused memory to grow, and not be recovered, in the off nominal
path (at least on my machine).

Change-Id: I22ee65e1e1bff8efacea8a167c6c8428898523f7