I'm not a 100% sure, but maybe we should we set a flag in `NgxBaseFetch::Release()` indicating that the base fetch is released, so we can use that to prevent an in-flight `NgxBaseFetch::HandleDone` from signalling a released base_fetch in between `base_fetch->Release()` and `ps_base_fetch_clear()`? 

@jeffkaufman what do you think?