mirb dies if #inspect returns a non-string value

```sh
% echo 'class A; def inspect; 1; end; end; A.new' | bin/mirb
mirb - Embeddable Interactive Ruby Shell

> 
zsh: segmentation fault (core dumped)  bin/mirb
```

`Kernel#p` shows nothing if `Object#inspect` returns a non-string value.  mirb should do that too.