fix proxy round tripper race condition

proxy/round_tripper/proxy_round_tripper.go

@@ -280,11 +280,12 @@ func (rt *roundTripper) timedRoundTrip(tr http.RoundTripper, request *http.Reque
 
 	// unfortunately if the cancel function above is not called that
 	// results in a vet error
+	vrid := request.Header.Get(handlers.VcapRequestIdHeader)
 	go func() {
 		select {
 		case <-reqCtx.Done():
 			if reqCtx.Err() == context.DeadlineExceeded {
-				logger.Error("backend-request-timeout", zap.Error(reqCtx.Err()), zap.String("vcap_request_id", request.Header.Get(handlers.VcapRequestIdHeader)))
+				logger.Error("backend-request-timeout", zap.Error(reqCtx.Err()), zap.String("vcap_request_id", vrid))
 			}
 			cancel()
 		}