Avoid nil dereference if passing a nil resolver

Signed-off-by: Antonio Gamez Diaz <agamez@vmware.com>

pkg/registry/client.go

@@ -88,8 +88,15 @@ func NewClient(options ...ClientOption) (*Client, error) {
 		client.authorizer = authClient
 	}
 
-	if client.resolver == nil {
+	resolverFn := client.resolver // copy for avoiding recursive call
 	client.resolver = func(ref registry.Reference) (remotes.Resolver, error) {
+		if resolverFn != nil {
+			// validate if the resolverFn returns a valid resolver
+			if resolver, err := resolverFn(ref); resolver != nil && err == nil {
+				return resolver, nil
+			}
+		}
+
 		headers := http.Header{}
 		headers.Set("User-Agent", version.GetUserAgent())
 		dockerClient, ok := client.authorizer.(*dockerauth.Client)
@@ -119,7 +126,6 @@ func NewClient(options ...ClientOption) (*Client, error) {
 		}
 		return resolver, nil
 	}
-	}
 
 	// allocate a cache if option is set
 	var cache registryauth.Cache