applayer: error if probes are null but port is not

If the default port is set via the Rust registration table but the probe
fns to server and to client are set to None, the port is never used.
Setting port in such a case is useless so error out.

src/app-layer-register.c

@@ -55,6 +55,7 @@ AppProto AppLayerRegisterProtocolDetection(const struct AppLayerParser *p, int e
     AppLayerProtoDetectRegisterProtocol(alproto, p->name);
 
     if (p->ProbeTS == NULL && p->ProbeTC == NULL) {
+        BUG_ON(p->default_port != NULL);
         return alproto;
     }