Fix thread-safety bug in Obj.getAllKeys

[JoshRosen]

This PR fixes a thread-safety bug in Val.obj.getAllKeys, similar to the previous fix in #136. It looks like this particular site was overlooked in that earlier PR.

I spotted this potential issue while reading through the code, not via actual use.

[JoshRosen]

On reflection, I'm actually unsure about whether this race condition can occur in practice:

If the parse cache is the means by which objects can be shared across threads and that can only cache static objects, then:

• Static objects have allKeys pre-assigned,
• thus we'll never attempt to initialize allKeys,
• thus we'll not hit a race condition here.

In contrast, I think that getValue0 could potentially be invoked for a cached static object if addSuper is called, which could happen if we're using the binary + operation for object-object concatenation.