Fix union.test failure in buildbot: alternate fix for BUG#49734

mysql-test/r/union.result:
  Update test results (checked, it should be "DEPENDENT SUBQUERY" as it will actually run subquery's join multiple times. There is no reason to do that but executor will still do it)

mysql-test/r/union.result

@@ -1632,7 +1632,7 @@ ORDER BY (SELECT a FROM t2 WHERE b = 12);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	
 2	UNION	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	
-3	SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
+3	DEPENDENT SUBQUERY	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
 NULL	UNION RESULT	<union1,2>	ALL	NULL	NULL	NULL	NULL	NULL	NULL	Using filesort
 Warnings:
 Note	1276	Field or reference 'test.t1.a' of SELECT #3 was resolved in SELECT #2

sql/sql_select.cc

@@ -18319,6 +18319,26 @@ static void select_describe(JOIN *join, bool need_tmp_table, bool need_order,
        unit;
        unit= unit->next_unit())
   {
+    /*
+      This fix_fields() call is to handle an edge case like this:
+       
+        SELECT ... UNION SELECT ... ORDER BY (SELECT ...)
+      
+      for such queries, we'll get here before having called
+      subquery_expr->fix_fields(), which will cause failure to
+    */
+    if (unit->item && !unit->item->fixed)
+    {
+      Item *ref= unit->item;
+      if (unit->item->fix_fields(thd, &ref))
+        DBUG_VOID_RETURN;
+      DBUG_ASSERT(ref == unit->item);
+    }
+
+    /* 
+      Display subqueries only if they are not parts of eliminated WHERE/ON
+      clauses.
+    */
     if (!(unit->item && unit->item->eliminated))
     {
       if (mysql_explain_union(thd, unit, result))