Additional sanity checks:

- don't sample a dead isolate
- only try to retrieve an async ID when there's a context

Author: szegedi

bindings/profilers/wall.cc

@@ -291,6 +291,9 @@ void SignalHandler::HandleProfilerSignal(int sig,
     return;
   }
   auto isolate = Isolate::GetCurrent();
+  if (!isolate || isolate->IsDead()) {
+    return;
+  }
   WallProfiler* prof = g_profilers.GetProfiler(isolate);
 
   if (!prof) {

bindings/profilers/wall.hh

@@ -155,14 +155,18 @@ class WallProfiler : public Nan::ObjectWrap {
   int64_t GetAsyncId(v8::Isolate* isolate) const {
     if (gcCount > 0) {
       return gcAsyncId;
+    } else if (isolate->InContext()) {
+      return static_cast<int64_t>(node::AsyncHooksGetExecutionAsyncId(isolate));
     }
-    return static_cast<int64_t>(node::AsyncHooksGetExecutionAsyncId(isolate));
+    return -1;
   }
 
   void OnGCStart(v8::Isolate* isolate) {
-    if (gcCount++ == 0) {
-      gcAsyncId =
-          static_cast<int64_t>(node::AsyncHooksGetExecutionAsyncId(isolate));
+    if (gcCount == 0) {
+      gcAsyncId = GetAsyncId(isolate);
+      gcCount = 1;
+    } else {
+      ++gcCount;
     }
   }