fs: fix potential segfault in async calls

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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)