Bug#20034943 Assertion failed: argument_count() > 0

A refactoring removed an apparently irrelevant if test and an
indentation level from SELECT_LEX::change_group_ref(), but this bug
proves that the if test had a mission.

Fixed by calling Item::arguments() only inside the loop, hence
eliminating the call when arg_count is zero.

Author: roylyseng

mysql-test/r/olap.result

@@ -744,3 +744,12 @@ f1
 11:11:11
 DROP TABLE t1;
 End of 5.5 tests
+# Bug#20034943 Assertion failed: argument_count() > 0
+CREATE TABLE t1(a INTEGER) engine=innodb;
+SELECT NOW() FROM t1 GROUP BY (select 1) WITH ROLLUP;
+NOW()
+DROP TABLE t1;
+CREATE TABLE t1(a INTEGER) engine=innodb;
+SELECT RELEASE_ALL_LOCKS() FROM t1 GROUP BY a WITH ROLLUP;
+RELEASE_ALL_LOCKS()
+DROP TABLE t1;

mysql-test/t/olap.test

@@ -455,3 +455,13 @@ DROP TABLE t1;
 --source include/restore_sql_mode_after_turn_off_only_full_group_by.inc
 
 --echo End of 5.5 tests
+
+--echo # Bug#20034943 Assertion failed: argument_count() > 0
+
+CREATE TABLE t1(a INTEGER) engine=innodb;
+SELECT NOW() FROM t1 GROUP BY (select 1) WITH ROLLUP;
+DROP TABLE t1;
+
+CREATE TABLE t1(a INTEGER) engine=innodb;
+SELECT RELEASE_ALL_LOCKS() FROM t1 GROUP BY a WITH ROLLUP;
+DROP TABLE t1;

sql/sql_resolver.cc

@@ -3363,12 +3363,10 @@ bool SELECT_LEX::setup_group(THD *thd)
 
 bool SELECT_LEX::change_group_ref(THD *thd, Item_func *expr, bool *changed)
 {
-  Item **arg,**arg_end;
   bool arg_changed= false;
-  for (arg= expr->arguments(),
-       arg_end= expr->arguments()+expr->arg_count;
-       arg != arg_end; arg++)
+  for (uint i= 0; i < expr->arg_count; i++)
   {
+    Item **arg= expr->arguments() + i;
     Item *const item= *arg;
     if (item->type() == Item::FIELD_ITEM || item->type() == Item::REF_ITEM)
     {