Fix null dereference in GC_stack_range_for if not DARWIN_DONT_PARSE_STACK

Fix null dereference in GC_stack_range_for if not DARWIN_DONT_PARSE_STACK
(fix commit 4ceae609)

* darwin_stop_world.c (GC_stack_range_for): Use GC_ATTR_UNUSED for
paltstack_hi argument (because the latter is not used currently if
!DARWIN_DONT_PARSE_STACK); do not access p->altstack[_size] unless
DARWIN_DONT_PARSE_STACK; add TODO item.
* darwin_stop_world.c [!DARWIN_DONT_PARSE_STACK] (GC_push_all_stacks):
Do not use altstack_hi and altstack_lo; add TODO item.