Improve fix for macroexp crash with debugging.

Improve fix for macroexp crash with debugging.

* lisp.h (ASET) [ENABLE_CHECKING]: Pay attention to
ARRAY_MARK_FLAG when checking subscripts, because ASET is
not supposed to be invoked from the garbage collector.
See Andreas Schwab in <http://bugs.gnu.org/12118#25>.
(gc_aset): New function, which is like ASET but can be
used in the garbage collector.
(set_hash_key, set_hash_value, set_hash_next, set_hash_hash)
(set_hash_index): Use it instead of ASET.