internal/proxy: add missing defer to New

Change-Id: I4b1f5d89febd3094dd9e8d4c2c5d3acd3f5ea3de
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/239358
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/proxy/client.go b/internal/proxy/client.go
index 1d22489..438cb51 100644
--- a/internal/proxy/client.go
+++ b/internal/proxy/client.go
@@ -45,7 +45,7 @@
 // New constructs a *Client using the provided rawurl, which is expected to
 // be an absolute URI that can be directly passed to http.Get.
 func New(rawurl string) (_ *Client, err error) {
-	derrors.Wrap(&err, "proxy.New(%q)", rawurl)
+	defer derrors.Wrap(&err, "proxy.New(%q)", rawurl)
 	url, err := url.Parse(rawurl)
 	if err != nil {
 		return nil, fmt.Errorf("url.Parse: %v", err)