We obviously have broken pointer arithmetics that hands us
glr_stack_items that are not glr_stack_items. Have a simple check for
this, to have earlier failures.
* data/skeletons/glr2.cc (glr_stack_item::check_): New.
Use it.
(glr_stack_item::contents): Avoid the useless struct.
Fix minor stylistic issues.
2313f891bd