MDL-62619 privacy: Fix a missing alias in the blog provider class

The new implementation revealed another existing bug that did not expose
before due to coincidently same named table alias in the outer query.

blog/classes/privacy/provider.php

@@ -135,7 +135,7 @@ class provider implements
         if ($DB->record_exists('blog_external', ['userid' => $userid])) {
             $sql = "
                 SELECT ctx.id
-                  FROM {context}
+                  FROM {context} ctx
                  WHERE ctx.contextlevel = :ctxlevel
                    AND ctx.instanceid = :ctxuserid";
             $params = [