def_load_bio(): Free |biosk| more carefully

If there's anything in the |biosk| stack, the first element is always
the input BIO.  It should never be freed in this function, so we must
take careful steps not to do so inadvertently when freeing the stack.

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/6769)