registry: use len(via)!=0 instead of via!=nil

This avoids the corner case where `via` is not nil, but has a length of 0,
so the updated code does not panic in that situation.

Signed-off-by: Iskander Sharipov <quasilyte@gmail.com>

registry/registry.go

@@ -145,7 +145,7 @@ func trustedLocation(req *http.Request) bool {
 // addRequiredHeadersToRedirectedRequests adds the necessary redirection headers
 // for redirected requests
 func addRequiredHeadersToRedirectedRequests(req *http.Request, via []*http.Request) error {
-	if via != nil && via[0] != nil {
+	if len(via) != 0 && via[0] != nil {
 		if trustedLocation(req) && trustedLocation(via[0]) {
 			req.Header = via[0].Header
 			return nil