Modify handling of `bodystmt`.

In the current implementation, segmentation fault occurs
if `bodystmt` has `opt_else` without `opt_rescue`.

For example, the following code causes segmentation fault.
```ruby
begin
  p 1
else
  p 2
end
```

I think that `opt_else` without `opt_rescue` is useless, but segmentation fault should not be caused.