Check bucket->object_space for existence in finalizer

Overriden method might raise exception before the bucket will reach
initialize method, for example if user will do something like this:

    class Couchbase::Bucket
      def initialize(*args)
        raise "something wrong"
        super
      end
    end

Change-Id: Ia7b00db0834f1bb90064370fb7b0dddf369219f3
Reviewed-on: http://review.couchbase.org/32332
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>