bug#36434: 26.2; smie-rule-next-p needs smie--after to be defined

[Sam Halliday]

When writing SMIE indentation rules, as per `smie-rules-function`,
when an :elem is received (confirmed at least with 'empty-line-token)
it is not possible to call the function smie-rule-parent-p, because
the smie--after variable has not been defined.

A workaround is to define the variable locally, e.g.

        (defvar smie--after)
        (setq smie--after (point))