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]

1 files changed, 7 insertions, 0 deletions

diff --git a/test/objspace/test_objspace.rb b/test/objspace/test_objspace.rb
index eabfff7876..d9b162ad2b 100644
--- a/test/objspace/test_objspace.rb
+++ b/test/objspace/test_objspace.rb
@@ -224,6 +224,13 @@ class TestObjSpace < Test::Unit::TestCase
         1.0 / 0.0; line4 = __LINE__; c4 = GC.count
       assert_equal(__FILE__, ObjectSpace.allocation_sourcefile(o4))
       assert_equal(line4, ObjectSpace.allocation_sourceline(o4))
+
+      # [Bug #19482]
+      EnvUtil.under_gc_stress do
+        100.times do
+          Class.new
+        end
+      end
     }
   end