target_class should not be TT_ICLASS in instance_eval(string); ref #1152

when mrb_funcall called instance_eval(string), mrb->c->ci->target_class doesn't modified.

```c
mrb_funcall(mrb, mrb_obj_value(mrb->kernel_module), "instance_eval", 1, "class Foo; end");
// ==> TypeError: #<Kernel:0x8a256a0> is not a class/module
```

target_class should not be TT_ICLASS. ref #1152