Fix type error in Go 1.3

Suggested by the poster at https://github.com/sourcegraph/go-webkit2/issues/12#issuecomment-47535929/

Fixes a type error introduced by changes in Go 1.3.

```
$ go test ./...
# github.com/crazy2be/gojs
../../crazy2be/gojs/context.go:13: cannot use (*[0]uint8)(c_nil) (type *[0]uint8) as type *C.struct_OpaqueJSClass in argument to _Cfunc_JSGlobalContextCreate

```

As far as I can tell, this change is necessary for gojs to compile in Go 1.3.