portable method to zeroize memory when destroying a mpz number

[Vicente Benjumea]

Hi,

when implementing public key cryptography algorithms, it would be nice
to have a portable method to zeroize (reset to zero) the memory that
holds the big-number bits before being destroyed, specially for private
keys.

For example, something similar to:

void
mpz_clear_zeroize (mpz_ptr m)
{
  if (PTR (m) != NULL) {
    memset(PTR (m), 0, ALLOC (m) * BYTES_PER_MP_LIMB);
  }
  (*__gmp_free_func) (PTR (m), ALLOC (m) * BYTES_PER_MP_LIMB);
  ALLOC (m) = 0;
  PTR (m) = 0;
  SIZ (m) = 0;
}

if this method appears in the public interface of GMP (mpz), then it can
be safely used even if the current internal implementation changes.

Thank you very much
Vicente