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
```

1 files changed, 1 insertions, 1 deletions

diff --git a/variable.c b/variable.c
index 3d70d7b808..511d5c7d54 100644
--- a/variable.c
+++ b/variable.c
@@ -1399,12 +1399,12 @@ rb_obj_convert_to_too_complex(VALUE obj, st_table *table)
 
             struct gen_ivtbl *ivtbl = xmalloc(sizeof(struct gen_ivtbl));
             ivtbl->as.complex.table = table;
+            st_insert(gen_ivs, (st_data_t)obj, (st_data_t)ivtbl);
 #if SHAPE_IN_BASIC_FLAGS
             rb_shape_set_shape_id(obj, OBJ_TOO_COMPLEX_SHAPE_ID);
 #else
             ivtbl->shape_id = OBJ_TOO_COMPLEX_SHAPE_ID;
 #endif
-            st_insert(gen_ivs, (st_data_t)obj, (st_data_t)ivtbl);
         }
         RB_VM_LOCK_LEAVE();
     }