sys: rtpbuffer: onebyte and twobyte get functions have wrong signature

The return data pointer is marked as data: *mut u8 but it is a pointer to a pointer, so we need to do ugly pointer management on rust side to be able to use it.

let mut data = ptr::null_mut();
let data_ptr = &mut data as *mut *mut u8 as *mut u8;

it would be easier if it took a *mut *mut u8 or some other way to make this easier to use and read.