protect against case with null sofia_private in callback

src/mod/endpoints/mod_sofia/sofia.c

@@ -1209,12 +1209,14 @@ static void our_sofia_event_callback(nua_event_t event,
 		sofia_handle_sip_i_options(status, phrase, nua, profile, nh, sofia_private, sip, de, tags);
 		break;
 	case nua_i_invite:
-		if (session && sofia_private->is_call > 1) {
+		if (session && sofia_private) {
+			if (sofia_private->is_call > 1) {
 				sofia_handle_sip_i_reinvite(session, nua, profile, nh, sofia_private, sip, de, tags);
 			} else {
 				sofia_private->is_call++;
 				sofia_handle_sip_i_invite(session, nua, profile, nh, sofia_private, sip, de, tags);
 			}
+		}
 		break;
 	case nua_i_publish:
 		sofia_presence_handle_sip_i_publish(nua, profile, nh, sofia_private, sip, de, tags);