MDEV-17942 fixup : protect rebuild_check_host() / rebuild_role_grants() with acl_cache->lock mutex

sql/sql_acl.cc

@@ -9573,16 +9573,17 @@ bool mysql_create_user(THD *thd, List <LEX_USER> &list, bool handle_as_role)
     }
   }
 
+  if (result && some_users_dropped && !handle_as_role)
+  {
+    /* Rebuild in-memory structs, since 'acl_users' has been modified */
+    rebuild_check_host();
+    rebuild_role_grants();
+  }
+
   mysql_mutex_unlock(&acl_cache->lock);
 
   if (result)
   {
-    if (some_users_dropped && !handle_as_role)
-    {
-      /* Rebuild in-memory structs, since 'acl_users' has been modified */
-      rebuild_check_host();
-      rebuild_role_grants();
-    }
     my_error(ER_CANNOT_USER, MYF(0),
              (handle_as_role) ? "CREATE ROLE" : "CREATE USER",
              wrong_users.c_ptr_safe());