zv: More robust check for non-unit enum deserialization

We were checking for a ) to check if we just deserialized a non-unit enum but this would also be true if we just deserialized a unit enum and we're at the end of a containing structure (i-e a unit enum at the end of a structure). This meant that we prematurely parse the ) and if this happens to be end of a message, we get a crash when the structure deserializer tries to parse the signature still (as it is still expecting at least a )).

Fixes #265 (closed)