ENH: reset tmp via assignment from literal nullptr (#1775)

- previously this was marked as '= delete' for consistency with
  assignment from an empty pointer being a runtime error.
  However, these can be considered semantically different and it makes
  sense to permit this as equivalent to reset(nullptr).

  This change does not break existing code since the operator was
  previously unavailable (deleted).

STYLE: refactor tmp operator=(T*)

- delegate to reset() after initial checks