Check the correct error in d.downloadAPIs

The error result of `d.downloadAPIs()` is set in `aerr`,
not `err`.

This prevents a nil-ptr dereference of apiGroups in the next step.

Signed-off-by: Abhijit Hoskeri <abhijithoskeri@gmail.com>

Kubernetes-commit: f8b99b1f09fb5d4d10b15e326c4b242cc705f007

discovery/discovery_client.go

@@ -196,7 +196,7 @@ func (d *DiscoveryClient) GroupsAndMaybeResources() (*metav1.APIGroupList, map[s
 	}
 	// Discovery groups and (possibly) resources downloaded from /apis.
 	apiGroups, apiResources, aerr := d.downloadAPIs()
-	if err != nil {
+	if aerr != nil {
 		return nil, nil, aerr
 	}
 	// Merge apis groups into the legacy groups.