src: fix nullptr access on realm

Adding the per-realm binding data tracking introduced a call to a realm
method when realm was potentially null. This is triggered whenever the
function is called from `ContextifyContext::New` which passes `nullptr`
as the `realm`.

src/env.cc

@@ -575,7 +575,7 @@ void Environment::AssignToContext(Local<v8::Context> context,
   // Used to retrieve bindings
   context->SetAlignedPointerInEmbedderData(
       ContextEmbedderIndex::kBindingDataStoreIndex,
-      realm->binding_data_store());
+      realm != nullptr ? realm->binding_data_store() : nullptr);
 
   // ContextifyContexts will update this to a pointer to the native object.
   context->SetAlignedPointerInEmbedderData(