Avoid a double-free in an error path.

Signed-off-by: NKurt Roeckx <kurt@roeckx.be>
Reviewed-by: NMatt Caswell <matt@openssl.org>

crypto/dh/dh_ameth.c

@@ -283,6 +283,7 @@ static int dh_priv_encode(PKCS8_PRIV_KEY_INFO *p8, const EVP_PKEY *pkey)
     dplen = i2d_ASN1_INTEGER(prkey, &dp);
 
     ASN1_INTEGER_free(prkey);
+    prkey = NULL;
 
     if (!PKCS8_pkey_set0(p8, OBJ_nid2obj(pkey->ameth->pkey_id), 0,
                          V_ASN1_SEQUENCE, params, dp, dplen))