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()

mysql-test/t/subselect4.test

@@ -62,3 +62,32 @@ FROM t3 WHERE 1 = 0 GROUP BY 1;
 DROP TABLE t1,t2,t3;
 
 --echo End of 5.0 tests.
+
+--echo #
+--echo # Bug#53236 Segfault in DTCollation::set(DTCollation&)
+--echo #
+
+CREATE TABLE t1 (
+  pk INTEGER AUTO_INCREMENT,
+  col_varchar VARCHAR(1),
+  PRIMARY KEY (pk)
+)
+;
+
+INSERT INTO t1 (col_varchar) 
+VALUES
+('w'),
+('m')
+;
+
+SELECT  table1.pk
+FROM ( t1 AS table1 JOIN t1 AS table2 ON (table1.col_varchar =
+                                          table2.col_varchar) ) 
+WHERE ( 1, 2 ) IN ( SELECT SUBQUERY1_t1.pk AS SUBQUERY1_field1,
+                           SUBQUERY1_t1.pk AS SUBQUERY1_field2
+                    FROM ( t1 AS SUBQUERY1_t1 JOIN t1 AS SUBQUERY1_t2
+                           ON (SUBQUERY1_t2.col_varchar =
+                               SUBQUERY1_t1.col_varchar) ) ) 
+;
+
+drop table t1;