Fix ordering for auto compaction in get_overloaded_cme()

Found through GC.stress + GC.auto_compact crashes in GH-8932.
Previously, the compaction run within `rb_method_entry_alloc()` could
move the `def->body.iseq.cref` and `iseqptr` set up before the call and
leave the `def` pointing to moved addresses. Nothing was marking `def`
during that GC run.

Low probability reproducer:

    GC.stress = true
    GC.auto_compact = true
    arr = []
    alloc = 1000.times.map { [] }
    alloc = nil
    a = arr.first
    GC.start

0 files changed, 0 insertions, 0 deletions