item_func.cc:

  Post fix for bug#28494.
  The Item_func_set_user_var::check method now silently doesn't use result_field
  if it isn't defined.

sql/item_func.cc

@@ -4063,8 +4063,8 @@ bool
 Item_func_set_user_var::check(bool use_result_field)
 {
   DBUG_ENTER("Item_func_set_user_var::check");
-  if (use_result_field)
-    DBUG_ASSERT(result_field);
+  if (use_result_field && !result_field)
+    use_result_field= FALSE;
 
   switch (cached_result_type) {
   case REAL_RESULT: