dnsdist: Prevent dangling TCP fd in case setupTCPDownstream() fails

Remove the closed socket descriptor from the sockets map.
Otherwise, if an exception occurs in setupTCPDownstream(), we might
try to use it and close it again later, not knowing it has been
reassigned to another socket or, worse, to a TCP acceptor pipe.

pdns/dnsdist-tcp.cc

@@ -358,6 +358,8 @@ void* tcpClientThread(int pipefd)
         if(!sendNonBlockingMsgLen(dsock, dq.len, ds->tcpSendTimeout, ds->remote, ds->sourceAddr, ds->sourceItf)) {
 	  vinfolog("Downstream connection to %s died on us, getting a new one!", ds->getName());
           close(dsock);
+          dsock=-1;
+          sockets.erase(ds->remote);
           sockets[ds->remote]=dsock=setupTCPDownstream(ds);
           downstream_failures++;
           goto retry;
@@ -374,6 +376,8 @@ void* tcpClientThread(int pipefd)
         catch(const runtime_error& e) {
           vinfolog("Downstream connection to %s died on us, getting a new one!", ds->getName());
           close(dsock);
+          dsock=-1;
+          sockets.erase(ds->remote);
           sockets[ds->remote]=dsock=setupTCPDownstream(ds);
           downstream_failures++;
           goto retry;
@@ -382,6 +386,8 @@ void* tcpClientThread(int pipefd)
         if(!getNonBlockingMsgLen(dsock, &rlen, ds->tcpRecvTimeout)) {
 	  vinfolog("Downstream connection to %s died on us phase 2, getting a new one!", ds->getName());
           close(dsock);
+          dsock=-1;
+          sockets.erase(ds->remote);
           sockets[ds->remote]=dsock=setupTCPDownstream(ds);
           downstream_failures++;
           goto retry;