Avoid panic when download request fails at connection stage

`RawBody()` may return nil if the connection failed (e.g. connection
refused).

pkg/client/download.go

@@ -30,8 +30,8 @@ func (c *Client) download(ctx context.Context, w io.Writer, url string, expectDi
 
 	resp, err := req.Get(url)
 
-	if resp != nil {
-		defer multierr.AppendInvoke(&err, multierr.Close(resp.RawBody()))
+	if !(resp == nil || resp.RawBody() == nil) {
+		defer multierr.AppendFunc(&err, resp.RawBody().Close)
 	}
 
 	if err := convertError(err, resp); err != nil {

pkg/client/download_test.go

@@ -51,6 +51,15 @@ func TestDownload(t *testing.T) {
 				Message:    `418`,
 			},
 		},
+		{
+			name: "connection error",
+			setup: func(t *testing.T, transport *httpmock.MockTransport) {
+				transport.RegisterResponder(http.MethodGet, "/conn/error",
+					httpmock.ConnectionFailure)
+			},
+			url:     "/conn/error",
+			wantErr: cmpopts.AnyError,
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			transport := newMockTransport(t)