Client panic when server sends 0 connect.ErrorCode

[joshcarp]

Describe the bug

The client panics when server sends 0 connect.ErrorCode

To Reproduce

func TestZeroCode(t *testing.T) {
	mux := http.NewServeMux()
	pluggableServer := &pluggablePingServer{
		ping: func(_ context.Context, _ *connect.Request[pingv1.PingRequest]) (*connect.Response[pingv1.PingResponse], error) {
			return nil, connect.NewError(0, errors.New("error"))
		},
	}
	mux.Handle(pingv1connect.NewPingServiceHandler(pluggableServer))
	server := httptest.NewServer(mux)
	t.Cleanup(server.Close)
	connectClient := pingv1connect.NewPingServiceClient(server.Client(), server.URL)
	connectResp, err := connectClient.Ping(context.Background(), connect.NewRequest(&pingv1.PingRequest{}))
	assert.NotNil(t, err)
	assert.Nil(t, connectResp)
}

Yields:

=== RUN   TestZeroCode
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x10043fcb0]

goroutine 7 [running]:
github.com/bufbuild/connect-go.(*connectUnaryClientConn).validateResponse(0x140001e8100, 0x14000222090)
	/Users/joshcarp/Documents/github.com/bufbuild/connect-go/protocol_connect.go:423 +0x900
github.com/bufbuild/connect-go.(*duplexHTTPCall).makeRequest(0x140001ce880)
	/Users/joshcarp/Documents/github.com/bufbuild/connect-go/duplex_http_call.go:257 +0xac
created by github.com/bufbuild/connect-go.(*duplexHTTPCall).ensureRequestMade.func1
	/Users/joshcarp/Documents/github.com/bufbuild/connect-go/duplex_http_call.go:233 +0x64


Process finished with the exit code 1

Environment (please complete the following information):

• connect-go v1.1.0