dh: Use SSH_STRING_FREE()

Fixes T183

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

src/dh.c

@@ -365,7 +365,7 @@ SSH_PACKET_CALLBACK(ssh_packet_client_dh_reply){
       goto error;
   }
   rc = ssh_dh_import_next_pubkey_blob(session, pubkey_blob);
-  ssh_string_free(pubkey_blob);
+  SSH_STRING_FREE(pubkey_blob);
   if (rc != 0) {
       goto error;
   }