Missing `JsonToken.END_OBJECT` for nested Protobuf Objects

[cowtowncoder]

(note: found in test written for #108)

Looks like there is missing END_OBJECT for some case(s) of nested Objects.

This was exposed by databind PR FasterXML/jackson-databind#4885 but probably not caused by it -- different deserializer path likely changes handling of unexpected null token (in place of closing END_OBJECT).