Backport of Bug#53236 Segfault in DTCollation::set(DTCollation&)

Don't call member functions for a NIL pointer.


mysql-test/r/subselect4.result:
  Add test case.
mysql-test/t/subselect4.test:
  Add test case.
sql/sql_select.cc:
  If the (virtual) member function clone_item() returns NULL,
  there is no substitution to be made, and we don't need to set the collation.
  The test was invoking Item_cache::clone_item()

1 files changed, 2 insertions, 4 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index cf8f258d08d..d9b6bff0663 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -8629,10 +8629,9 @@ change_cond_ref_to_const(THD *thd, I_List<COND_CMP> *save_list,
        left_item->collation.collation == value->collation.collation))
   {
     Item *tmp=value->clone_item();
-    tmp->collation.set(right_item->collation);
-    
     if (tmp)
     {
+      tmp->collation.set(right_item->collation);
       thd->change_item_tree(args + 1, tmp);
       func->update_used_tables();
       if ((functype == Item_func::EQ_FUNC || functype == Item_func::EQUAL_FUNC)
@@ -8653,10 +8652,9 @@ change_cond_ref_to_const(THD *thd, I_List<COND_CMP> *save_list,
             right_item->collation.collation == value->collation.collation))
   {
     Item *tmp= value->clone_item();
-    tmp->collation.set(left_item->collation);
-    
     if (tmp)
     {
+      tmp->collation.set(left_item->collation);
       thd->change_item_tree(args, tmp);
       value= tmp;
       func->update_used_tables();