Coverity detected invalid iterator in perfdhcp

This might be a false positive, but it could be double checked anyway:

In isc::​perfdhcp::​AvalancheScen::​resendPackets(isc::​perfdhcp::​ExchangeType): An invalid or past-the-end iterator is being used

        // get some things from previous retransmissions
        auto start_time = pkt->getTimestamp();
        int current_pkt_resent_cnt = 0;
        auto r_it = retrans.find(trans_id);
        if (r_it != retrans.end()) {
            start_time = (*start_times.find(trans_id)).second;
            current_pkt_resent_cnt = (*r_it).second;
        } else {
            start_times[trans_id] = start_time;
        }

Dereferencing iterator start_times->find(trans_id) though it is already past the end of its container.

CID 1533337