gost_ec_keyx: Check CTX data before it's really used

This should fix Coverity warning:

  *** CID 345243:  Null pointer dereferences  (REVERSE_INULL)
  /gost_ec_keyx.c: 681 in pkey_gost2018_decrypt()
  675        o  Q_eph is on the same curve as server public key;
  676
  677        o  Q_eph is not equal to zero point;
  678
  679        o  q * Q_eph is not equal to zero point.
  680     */
  >>>     CID 345243:  Null pointer dereferences  (REVERSE_INULL)
  >>>     Null-checking "data" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
  681         if (eph_key == NULL || priv == NULL || data == NULL) {
  682            GOSTerr(GOST_F_PKEY_GOST2018_DECRYPT,
  683                    GOST_R_ERROR_COMPUTING_EXPORT_KEYS);
  684            ret = 0;
  685            goto err;
  686         }

Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
Issue: #380