fix(ui): don't allow decor provider with ns_id==0

Fixes #19831

src/nvim/decoration_provider.c

@@ -174,6 +174,7 @@ void decor_providers_invoke_end(DecorProviders *providers, char **err)
 
 DecorProvider *get_decor_provider(NS ns_id, bool force)
 {
+  assert(ns_id > 0);
   size_t i;
   size_t len = kv_size(decor_providers);
   for (i = 0; i < len; i++) {

src/nvim/highlight.c

@@ -174,6 +174,11 @@ int ns_get_hl(NS *ns_hl, int hl_id, bool link, bool nodefault)
 {
   static int recursive = 0;
 
+  if (*ns_hl == 0) {
+    // ns=0 (the default namespace) does not have a provider so stop here
+    return -1;
+  }
+
   if (*ns_hl < 0) {
     if (ns_hl_active <= 0) {
       return -1;