When the async uv_fs_* call errors out synchronously in AsyncDestCall,
the after callbacks (e.g. AfterNoArgs) would delete the req_wrap
in FSReqAfterScope, and AsyncDestCall would set those req_wrap to
nullptr afterwards. But when it returns to the top-layer bindings,
the bindings all call `req_wrap->SetReturnValue()` again without
checking if `req_wrap` is nullptr, causing a segfault.

This has not been caught in any of the tests because we usually do a lot
of argument checking in the JS layer before invoking the uv_fs_*
functions, so it's rare to get a synchronous error from them.

Currently we never need the binding to return the wrap to JS layer,
so we can just call `req_wrap->SetReturnValue()` to return undefined for
normal FSReqWrap and the promise for FSReqPromise in AsyncDestCall instead
of doing this in the top-level bindings.