Fixes during review of new code

- Mostly indentation fixes
- Added missing test
- Ensure that Item_func_case() checks for stack overruns
- Use real_item() instead of (Item_ref*) item
- Fixed wrong error handling

mysql-test/t/group_by.test

@@ -542,6 +542,29 @@ SELECT hostname, COUNT(DISTINCT user_id) as no FROM t1
 
 DROP TABLE t1;
 
+#
+# Bug#11211: Ambiguous column reference in GROUP BY.
+#
+
+create table t1 (c1 char(3), c2 char(3));
+create table t2 (c3 char(3), c4 char(3));
+insert into t1 values ('aaa', 'bb1'), ('aaa', 'bb2');
+insert into t2 values ('aaa', 'bb1'), ('aaa', 'bb2');
+
+# query with ambiguous column reference 'c2'
+--disable_ps_protocol
+select t1.c1 as c2 from t1, t2 where t1.c2 = t2.c4
+group by c2;
+show warnings;
+--enable_ps_protocol
+
+# this query has no ambiguity
+select t1.c1 as c2 from t1, t2 where t1.c2 = t2.c4
+group by t1.c1;
+
+show warnings;
+drop table t1, t2;
+
 #
 # Test for bug #8614: GROUP BY 'const' with DISTINCT  
 #