clearing the ecx private key memory

Reviewed-by: NDmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9830)

(cherry picked from commit 6ef03ea98fac501e6d6e33bac6ad3c92ea074712)

crypto/ec/ecx_meth.c

@@ -191,7 +191,7 @@ static int ecx_priv_decode(EVP_PKEY *pkey, const PKCS8_PRIV_KEY_INFO *p8)
     }
 
     rv = ecx_key_op(pkey, pkey->ameth->pkey_id, palg, p, plen, KEY_OP_PRIVATE);
-    ASN1_OCTET_STRING_free(oct);
+    ASN1_STRING_clear_free(oct);
     return rv;
 }