async_hooks: check for empty contexts before removing

This way we don't end up attempting to SetPromiseHooks on contexts that
have already been collected.

Fixes: nodejs#39019

src/env-inl.h

@@ -256,8 +256,12 @@ inline void AsyncHooks::RemoveContext(v8::Local<v8::Context> ctx) {
   v8::Isolate* isolate = env()->isolate();
   v8::HandleScope handle_scope(isolate);
   for (auto it = contexts_.begin(); it != contexts_.end(); it++) {
+    if (it->IsEmpty()) {
+      contexts_.erase(it);
+      break;
+    }
     v8::Local<v8::Context> saved_context =
-      PersistentToLocal::Weak(env()->isolate(), *it);
+      PersistentToLocal::Weak(isolate, *it);
     if (saved_context == ctx) {
       it->Reset();
       contexts_.erase(it);

test/parallel/test-async-hooks-vm-gc.js

@@ -0,0 +1,15 @@
+// Flags: --expose-gc
+'use strict';
+
+require('../common');
+const asyncHooks = require('async_hooks');
+const vm = require('vm');
+
+// This is a regression test for https://github.com/nodejs/node/issues/39019
+//
+// It should not segfault.
+
+const hook = asyncHooks.createHook({ init() {} }).enable();
+vm.createContext();
+gc();
+hook.disable();