NPE when deserializing malformed collections from client

When deserializing collections sent from the client, if an element in the collection is incorrectly serialized, Collections.getValue can return null if the length of the element is negative. Currently this isn't detected and serialization continues, calling validate and throwing an NPE in serializers that don't handle null value buffers.

Detect the malformed input and throw a better MarshalException so it will be converted to an InvalidRequestException for the client.