Bug #30942: select str_to_date from derived table returns varying results

The function str_to_date has a field to say whether it's invoked constant 
arguments. But this member was not initialized, causing the function to
think that it could use a cache of the format type when said cache was in 
fact not initialized.
Fixed by initializing the field to false.

mysql-test/r/type_date.result

@@ -136,3 +136,13 @@ d	dt	ts
 0000-00-00	0000-00-00 00:00:00	0000-00-00 00:00:00
 2001-11-11	2001-11-11 00:00:00	2001-11-11 00:00:00
 drop table t1;
+CREATE TABLE t1 (
+a INT
+);
+INSERT INTO t1 VALUES (1);
+INSERT INTO t1 VALUES (NULL);
+SELECT str_to_date( '', a ) FROM t1;
+str_to_date( '', a )
+0000-00-00 00:00:00
+NULL
+DROP TABLE t1;

mysql-test/t/type_date.test

@@ -136,3 +136,16 @@ insert into t1 values (9912101,9912101,9912101);
 insert into t1 values (11111,11111,11111);
 select * from t1;
 drop table t1;
+
+#
+# Bug #30942: select str_to_date from derived table returns varying results
+#
+CREATE TABLE t1 (
+  a INT
+);
+
+INSERT INTO t1 VALUES (1);
+INSERT INTO t1 VALUES (NULL);
+
+SELECT str_to_date( '', a ) FROM t1;
+DROP TABLE t1;

sql/item_timefunc.h

@@ -1030,7 +1030,7 @@ class Item_func_str_to_date :public Item_str_func
   bool const_item;
 public:
   Item_func_str_to_date(Item *a, Item *b)
-    :Item_str_func(a, b)
+    :Item_str_func(a, b), const_item(false)
   {}
   String *val_str(String *str);
   bool get_date(MYSQL_TIME *ltime, uint fuzzy_date);