Fixed bug #26661: crash when order by clause in a union

construct references invalid name.
Derived tables currently cannot use outer references.
Thus there is no outer context for them.
The 4.1 code takes this fact into account while the 
Item_field::fix_outer_field code of 5.0 lost the check that blocks
any attempts to resolve names in outer context for derived tables.

mysql-test/r/union.result

@@ -1370,4 +1370,15 @@ select _utf8'12' union select _latin1'12345';
 12
 12
 12345
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (3),(1),(2),(4),(1);
+SELECT a FROM (SELECT a FROM t1 UNION SELECT a FROM t1 ORDER BY a) AS test;
+a
+1
+2
+3
+4
+SELECT a FROM (SELECT a FROM t1 UNION SELECT a FROM t1 ORDER BY c) AS test;
+ERROR 42S22: Unknown column 'c' in 'order clause'
+DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/union.test

@@ -855,4 +855,17 @@ drop table t1, t2;
 #
 select _utf8'12' union select _latin1'12345';
 
+#
+# Bug #26661: UNION with ORDER BY undefined column in FROM list
+#
+
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (3),(1),(2),(4),(1);
+
+SELECT a FROM (SELECT a FROM t1 UNION SELECT a FROM t1 ORDER BY a) AS test;
+--error 1054 
+SELECT a FROM (SELECT a FROM t1 UNION SELECT a FROM t1 ORDER BY c) AS test;
+
+DROP TABLE t1;
+
 --echo End of 5.0 tests

sql/item.cc

@@ -3441,7 +3441,12 @@ Item_field::fix_outer_field(THD *thd, Field **from_field, Item **reference)
   */
   Name_resolution_context *last_checked_context= context;
   Item **ref= (Item **) not_found_item;
-  Name_resolution_context *outer_context= context->outer_context;
+  SELECT_LEX *current_sel= (SELECT_LEX *) thd->lex->current_select;
+  Name_resolution_context *outer_context= 0;
+  /* Currently derived tables cannot be correlated */
+  if (current_sel->master_unit()->first_select()->linkage !=
+      DERIVED_TABLE_TYPE)
+    outer_context= context->outer_context;
   for (;
        outer_context;
        outer_context= outer_context->outer_context)