Don't SEGFAULT when trying to export a public DSA key as a private key.

crypto/dsa/dsa_ameth.c

@@ -307,6 +307,12 @@ static int dsa_priv_encode(PKCS8_PRIV_KEY_INFO *p8, const EVP_PKEY *pkey)
 	unsigned char *dp = NULL;
 	int dplen;
 
+	if (!pkey->pkey.dsa->priv_key)
+		{
+		DSAerr(DSA_F_DSA_PRIV_ENCODE,DSA_R_MISSING_PARAMETERS);
+		goto err;
+		}
+
 	params = ASN1_STRING_new();
 
 	if (!params)