Fix crash when allocating classes with newobj hook

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:

```
require "objspace"

GC.stress = true

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

[Bug #19482]

0 files changed, 0 insertions, 0 deletions