🐛 Fixed issue where single letter slugs for products would cause 500 …

…errors

refs TryGhost/Product#3224

When a product has a slug that is a single letter, checking if a user had
access to view a post associated with that product would cause a 500 error.
The underlying cause of this issue is TryGhost/NQL#20
This fix circumvents this issue by providing a value that the nql lexer
will not error out on

ghost/core/core/server/services/members/content-gating.js

@@ -46,7 +46,7 @@ function checkPostAccess(post, member) {
             return BLOCK_ACCESS;
         }
         visibility = post.tiers.map((product) => {
-            return `product:${product.slug}`;
+            return `product:'${product.slug}'`;
         }).join(',');
     }