Ftp::Gateway may segfault in level-3 double-complete debugs() (#1923)

Ftp::Gateway::completeForwarding() must check data.conn pointer before
dereferencing it. Long-term, we should improve Comm::ConnectionPointer
printing to safely report Connection::id (where available). This minimal
fix just mimics existing Ftp::Relay::abortOnData() solution.

src/clients/FtpGateway.cc

@@ -2639,7 +2639,7 @@ Ftp::Gateway::completeForwarding()
 {
     if (fwd == nullptr || flags.completed_forwarding) {
         debugs(9, 3, "avoid double-complete on FD " <<
-               (ctrl.conn ? ctrl.conn->fd : -1) << ", Data FD " << data.conn->fd <<
+               (ctrl.conn ? ctrl.conn->fd : -1) << ", Data FD " << (data.conn ? data.conn->fd : -1) <<
                ", this " << this << ", fwd " << fwd);
         return;
     }