Fix nil pointer reference in ServeDNS() with concurrent go routines.

Signed-off-by: Santhosh Manohar <santhosh@docker.com>

libnetwork/resolver.go

@@ -324,6 +324,13 @@ func (r *resolver) ServeDNS(w dns.ResponseWriter, query *dns.Msg) {
 					continue
 				}
 			}
+			// If two go routines are executing in parralel one will
+			// block on the Once.Do and in case of error connecting
+			// to the external server it will end up with a nil err
+			// but extConn also being nil.
+			if extConn == nil {
+				continue
+			}
 
 			// Timeout has to be set for every IO operation.
 			extConn.SetDeadline(time.Now().Add(extIOTimeout))