Previously, passing objects that respond to #to_int to `String#setbyte`
resulted in a crash when compiled by YJIT. This was due to the lazily
pushed frame from rb_yjit_lazy_push_frame() lingering and not being
popped by an exception as expected.

The fix is to ensure that `ec->cfp` is restored to before the lazy frame
push in case the method call for conversion succeeds. Right now, this is
only for conversion to integers.

Found running `ruby/spec`.

* clarify comment

We just need to make sure `ec->cfp` is always preserved and this can
convert without rising when `raise` is true.