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 <[email protected]>
Issue: #380

gost_ec_keyx.c

@@ -630,16 +630,24 @@ static int pkey_gost2018_decrypt(EVP_PKEY_CTX *pctx, unsigned char *key,
                           size_t in_len)
 {
     const unsigned char *p = in;
-    struct gost_pmeth_data *data = EVP_PKEY_CTX_get_data(pctx);
-    EVP_PKEY *priv = EVP_PKEY_CTX_get0_pkey(pctx);
+    struct gost_pmeth_data *data;
+    EVP_PKEY *priv;
     PSKeyTransport_gost *pst = NULL;
     int ret = 0;
     unsigned char expkeys[64];
     EVP_PKEY *eph_key = NULL;
-    int pkey_nid = EVP_PKEY_base_id(priv);
+    int pkey_nid;
     int mac_nid = NID_undef;
     int iv_len = 0;
 
+    if (!(data = EVP_PKEY_CTX_get_data(pctx)) ||
+        !(priv = EVP_PKEY_CTX_get0_pkey(pctx))) {
+       GOSTerr(GOST_F_PKEY_GOST2018_DECRYPT, GOST_R_ERROR_COMPUTING_EXPORT_KEYS);
+       ret = 0;
+       goto err;
+    }
+    pkey_nid = EVP_PKEY_base_id(priv);
+
     switch (data->cipher_nid) {
     case NID_magma_ctr:
         mac_nid = NID_magma_mac;
@@ -678,7 +686,7 @@ static int pkey_gost2018_decrypt(EVP_PKEY_CTX *pctx, unsigned char *key,
 
    o  q * Q_eph is not equal to zero point.
 */
-    if (eph_key == NULL || priv == NULL || data == NULL) {
+    if (eph_key == NULL) {
        GOSTerr(GOST_F_PKEY_GOST2018_DECRYPT,
                GOST_R_ERROR_COMPUTING_EXPORT_KEYS);
        ret = 0;