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

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