[ruby-core:112721] [Ruby master Bug#19482] Fix crash when allocating classes with newobj hook

Issue #19482 has been reported by peterzhu2118 (Peter Zhu).

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Bug #19482: Fix crash when allocating classes with newobj hook
https://bugs.ruby-lang.org/issues/19482

* Author: peterzhu2118 (Peter Zhu)
* Status: Open
* Priority: Normal
* Backport: 2.7: DONTNEED, 3.0: DONTNEED, 3.1: DONTNEED, 3.2: REQUIRED
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GitHub PR: https://github.com/ruby/ruby/pull/7464

We need to zero out the whole slot when running the newobj hook for a newly allocated class because the slot could be filled with garbage, which would cause a crash if a GC runs inside of the newobj hook.

For example, the following script crashes:

```ruby
require "objspace"

GC.stress = true

ObjectSpace.trace_object_allocations {
  100.times do
    Class.new
  end
}
```



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