Fix a bug that an object in grn_expr is used after it is freed

It is caused when you specify column name including pseudo column name
such as _key by GRN_BULK. Groonga server users will not do the
usage. Rroonga users may use the usage. For example:

    expression.parse("true",
                     :default_column => "_id", # <- HERE
                     :syntax => :script)