nts: allow missing SIV support

When compiled with NTS support, don't require a SIV cipher to be always
supported (e.g. due to a different version of a library used for
building). Handle this case with a fatal message instead of crash.
Also, check the support early in the client unit test to prevent a hang.

nts_ntp_server.c

@@ -67,6 +67,8 @@ NNS_Initialise(void)
 
   server = Malloc(sizeof (struct NtsServer));
   server->siv = SIV_CreateInstance(SERVER_SIV);
+  if (!server->siv)
+    LOG_FATAL("Could not initialise SIV cipher");
 }
 
 /* ================================================== */