(constant -4 of type uintptr) overflows uintptr

TinyGo can't compile the the [pointer_unsafe.go](https://github.com/protocolbuffers/protobuf-go/blob/master/internal/impl/pointer_unsafe.go#L142) file from https://github.com/protocolbuffers/protobuf-go. I get the following error message:

```
unsafe.Sizeof(unsafe.Pointer(nil)) - unsafe.Sizeof((MessageState literal)) (constant -4 of type uintptr) overflows uintptr
```
