async_wrap: update utility methods for flexibility

Original PR: ayojs/ayo#82

> This makes some of the internal `AsyncWrap::MakeCallback()`
> utility wrappers throw rather than crash the process when
> the requested method is not present on the `this` object.

> Doing so makes it easier for future code to expose C++
> objects directly to userland, where JS code can overwrite
> or delete such methods.

> PR-URL: ayojs/ayo#82
> Reviewed-By: Stephen Belanger <[email protected]>

src/async-wrap-inl.h

@@ -50,8 +50,13 @@ inline v8::MaybeLocal<v8::Value> AsyncWrap::MakeCallback(
     const v8::Local<v8::String> symbol,
     int argc,
     v8::Local<v8::Value>* argv) {
-  v8::Local<v8::Value> cb_v = object()->Get(symbol);
-  CHECK(cb_v->IsFunction());
+  v8::Local<v8::Value> cb_v;
+  if (!object()->Get(object()->CreationContext(), symbol).ToLocal(&cb_v))
+    return v8::MaybeLocal<v8::Value>();
+  if (!cb_v->IsFunction()) {
+    env()->ThrowError("callback must be a function");
+    return v8::MaybeLocal<v8::Value>();
+  }
   return MakeCallback(cb_v.As<v8::Function>(), argc, argv);
 }
 
@@ -60,8 +65,13 @@ inline v8::MaybeLocal<v8::Value> AsyncWrap::MakeCallback(
     uint32_t index,
     int argc,
     v8::Local<v8::Value>* argv) {
-  v8::Local<v8::Value> cb_v = object()->Get(index);
-  CHECK(cb_v->IsFunction());
+  v8::Local<v8::Value> cb_v;
+  if (!object()->Get(object()->CreationContext(), index).ToLocal(&cb_v))
+    return v8::MaybeLocal<v8::Value>();
+  if (!cb_v->IsFunction()) {
+    env()->ThrowError("callback must be a function");
+    return v8::MaybeLocal<v8::Value>();
+  }
   return MakeCallback(cb_v.As<v8::Function>(), argc, argv);
 }