fix a couple gc-related continuations bugs

Thanks to Juhani Rantanen for the report.

* libguile/continuations.c (scm_make_continuation): Delay making the smob
  until the data is fully initialized. Fixes a bug whereby a GC in
  scm_vm_capture_continuations would catch the us with an undefined
  continuation->vm_conts, leading to marking badness.

* libguile/vm.c (vm_cont_free): Report the correct size to scm_gc_free.