cssprovider: Don't add empty rulesets

This change was lost during the parser changes, but the newly added test
exposes it.

And fix the expected error, the syntax has changed.

gtk/gtkcssprovider.c

@@ -631,6 +631,12 @@ css_provider_commit (GtkCssProvider *css_provider,
   GtkCssProviderPrivate *priv = gtk_css_provider_get_instance_private (css_provider);
   GSList *l;
 
+  if (ruleset->styles == NULL)
+    {
+      g_slist_free_full (selectors, (GDestroyNotify) _gtk_css_selector_free);
+      return;
+    }
+
   for (l = selectors; l; l = l->next)
     {
       GtkCssRuleset new;

testsuite/css/parser/border-crash-3.24.errors

@@ -1 +1 @@
-border-crash-3.24.css:3: error: GTK_CSS_PROVIDER_ERROR_SYNTAX
+border-crash-3.24.css:3:3-10: error: GTK_CSS_PARSER_ERROR_SYNTAX