ttrpc: fix the issue of marshaling on nil will crash the server

Since some type's Marshal() on nil will crash runtime, thus
it's should check 'resp' before marshaling on it. If it's
nil, return directly to avoid the server crash.

Signed-off-by: lifupan <lifupan@gmail.com>

services.go

@@ -21,6 +21,7 @@ import (
 	"io"
 	"os"
 	"path"
+	"unsafe"
 
 	"github.com/gogo/protobuf/proto"
 	"github.com/pkg/errors"
@@ -95,6 +96,10 @@ func (s *serviceSet) dispatch(ctx context.Context, serviceName, methodName strin
 		return nil, err
 	}
 
+	if isNil(resp) {
+		return nil, errors.New("ttrpc: marshal called with nil")
+	}
+
 	switch v := resp.(type) {
 	case proto.Message:
 		r, err := proto.Marshal(v)
@@ -154,3 +159,7 @@ func convertCode(err error) codes.Code {
 func fullPath(service, method string) string {
 	return "/" + path.Join(service, method)
 }
+
+func isNil(resp interface{}) bool {
+	return (*[2]uintptr)(unsafe.Pointer(&resp))[1] == 0
+}