Should not overwrite `ci->target_class` in `mrb_exec_irep()`.
For example, the following code behaved wrong:
```ruby
Object.instance_exec(1,2,3){|*a|
def foo
42
end
}
p foo()
```
MODIFY
src/vm.c
src/vm.c
@@ -511,7 +511,6 @@ mrb_exec_irep(mrb_state *mrb, mrb_value self, struct RProc *p)
mrb->c->stack[0] = self;
ci->proc = p;
- ci->target_class = MRB_PROC_TARGET_CLASS(p);
if (MRB_PROC_CFUNC_P(p)) {
return MRB_PROC_CFUNC(p)(mrb, self);
}

Should not overwrite `ci->target_class` in `mrb_exec_irep()`.

For example, the following code behaved wrong:

```ruby
Object.instance_exec(1,2,3){|*a|
  def foo
    42
  end
}
p foo()
```