Check whether object is valid in allocation_info_tracer_compact

When reference updating ObjectSpace.trace_object_allocations, we need to
check whether the object is valid or not because it does not mark the
object so the object may be dead. This can cause a segmentation fault
if the object is on a free heap page.

For example, the following script crashes:

    require "objspace"

    objs = []
    ObjectSpace.trace_object_allocations do
      1_000_000.times do
        objs << Object.new
      end
    end

    objs = nil

    # Free pages that the objs were on
    GC.start

    # Run compaction and check that it doesn't crash
    GC.compact

1 files changed, 6 insertions, 0 deletions

diff --git a/gc.c b/gc.c
index f4322417b6..c5277ee2c8 100644
--- a/gc.c
+++ b/gc.c
@@ -1724,6 +1724,12 @@ rb_objspace_garbage_object_p(VALUE obj)
     return rb_gc_impl_garbage_object_p(rb_gc_get_objspace(), obj);
 }
 
+bool
+rb_gc_pointer_to_heap_p(VALUE obj)
+{
+    return rb_gc_impl_pointer_to_heap_p(rb_gc_get_objspace(), (void *)obj);
+}
+
 /*
  *  call-seq:
  *     ObjectSpace._id2ref(object_id) -> an_object