Sacrifice this code to the KNF deities.
Add many missing error checks (probably not exhaustive, but a good start):
- make VKO_compute_key() no longer void so that it can return failure.
- fix unchecked allocations in too many routines to mention /-:
- fix unchecked BN operations in gost2001_do_sign(), gost2001_do_verify(),
  VKO_compute_key().
- fix the gost2001_do_sign() interface violation by having its sole caller
  free the BIGNUM it passes to that function by itself, instead of having
  the callee do this.

Reviewed (except for the last item) by Dmitry Eremin-Solenikov.