Fix crash when evacuating generic ivar

When transitioning generic instance variable objects to too complex, we
set the shape first before performing inserting the new gen_ivtbl. The
st_insert for the new gen_ivtbl could allocate and cause a GC. If that
happens, then it will crash because the object will have a too complex
shape but not yet be backed by a st_table.

This commit changes the order so that the insert happens first before
the new shape is set.

The following script reproduces the issue:

```
o = []
o.instance_variable_set(:@a, 1)

i = 0
o = Object.new
while RubyVM::Shape.shapes_available > 0
  o.instance_variable_set(:"@i#{i}", 1)
  i += 1
end

ary = 1_000.times.map { [] }

GC.stress = true
ary.each do |o|
  o.instance_variable_set(:@a, 1)
  o.instance_variable_set(:@b, 1)
end
```

0 files changed, 0 insertions, 0 deletions