res_pjsip_session: Added new function calls to avoid ABI issues.

Added two new functions (ast_sip_session_get_dialog and
ast_sip_session_get_pjsip_inv_state) that retrieve the dialog and the
pjsip_inv_state respectively from the pjsip_inv_session on the
ast_sip_session struct. This is due to pjproject adding a new field to
the pjsip_inv_session struct that caused crashes when trying to access
fields that were no longer where they were expected to be if a module
was compiled against a different version of pjproject.

Resolves: #145
(cherry picked from commit 23c4d21d1b)

res/res_pjsip_session.c

@@ -3641,6 +3641,28 @@ struct ast_sip_session *ast_sip_dialog_get_session(pjsip_dialog *dlg)
 	return session;
 }
 
+pjsip_dialog *ast_sip_session_get_dialog(const struct ast_sip_session *session)
+{
+	pjsip_inv_session *inv_session = session->inv_session;
+
+	if (!inv_session) {
+		return NULL;
+	}
+
+	return inv_session->dlg;
+}
+
+pjsip_inv_state ast_sip_session_get_pjsip_inv_state(const struct ast_sip_session *session)
+{
+	pjsip_inv_session *inv_session = session->inv_session;
+
+	if (!inv_session) {
+		return PJSIP_INV_STATE_NULL;
+	}
+
+	return inv_session->state;
+}
+
 /*! \brief Fetch just the Caller ID number in order of PAI, RPID, From */
 static int fetch_callerid_num(struct ast_sip_session *session, pjsip_rx_data *rdata, char *buf, size_t len)
 {