Avoid recursive instance variable inspections

This code causes an infinite loop. (and arena overflow)

```
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> class A
* attr_accessor :val
* end
 => A
> a = A.new
 => #<A:0x970c578>
> a.val = a
Segmentation fault
```

patched: 
```
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> class A
* attr_accessor :val
* end
 => A
> a = A.new
 => #<A:0x8992578>
> a.val = a
 => #<A:0x8992578 @val=#<A:0x8992578>>
```