src: fix crash on FSReqPromise destructor
We are deciding whether to end fs promises by checking
can_call_into_js() whereas in the FSReqPromise destructor we're
using the is_stopping() check. Though this may look as semantically
correct it has issues because though both values are modified before
termination on Environment::ExitEnv() and both are atomic they are not
synchronized together so it may happen that when reaching the destructor
call_into_js may be set to false whereas is_stopping remains
false causing the crash. Fix this by using the same checks everywhere.