Currently, yycompressStack expects the free items to be states only.
That's not the case.
Fixes 712 and 730 pass. 748 still fails, but later and
differently (heap-use-after-free).
* data/skeletons/glr2.cc (glr_stack_item::setState): New.
(glr_stack_item::yycompressStack): Use it.
* tests/glr-regression.at: Adjust.