Fix memory leak in pkcs12 -export

Reviewed-by: NAndy Polyakov <appro@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2676)

crypto/pkcs12/p12_mutl.c

@@ -204,6 +204,9 @@ int PKCS12_setup_mac(PKCS12 *p12, int iter, unsigned char *salt, int saltlen,
 {
     X509_ALGOR *macalg;
 
+    PKCS12_MAC_DATA_free(p12->mac);
+    p12->mac = NULL;
+
     if ((p12->mac = PKCS12_MAC_DATA_new()) == NULL)
         return PKCS12_ERROR;
     if (iter > 1) {