MDEV-22591 Debug build crashes on EXECUTE IMMEDIATE '... WHERE ?' USING IGNORE

Removing a wrong DBUG_ASSERT:
When Item_param gets "unfixed" in cleanup(), its "fixed" gets assigned
to false, while item_item keeps the value. So the assert was wrong.

Perhaps, instead of removing the assert, it was possible to reset
item_type to NO_VALUE in cleanup. But this is not very important:
it's implemented in 10.4 in a better way:
Item_param::is_fixed() always returns true and it does not need to be "unfixed".

mysql-test/r/ps.result

@@ -5373,5 +5373,22 @@ drop procedure p;
 drop view v1;
 drop table t1;
 #
+# MDEV-22591 Debug build crashes on EXECUTE IMMEDIATE '... WHERE ?' USING IGNORE
+#
+CREATE TABLE t1 (a INT);
+EXECUTE IMMEDIATE 'SELECT * FROM t1 WHERE ?' USING IGNORE;
+ERROR HY000: Default/ignore value is not supported for such parameter usage
+EXECUTE IMMEDIATE 'SELECT * FROM t1 HAVING ?' USING IGNORE;
+ERROR HY000: Default/ignore value is not supported for such parameter usage
+EXECUTE IMMEDIATE 'SELECT * FROM t1 WHERE ?' USING 0;
+a
+EXECUTE IMMEDIATE 'SELECT * FROM t1 HAVING ?' USING 0;
+a
+DROP TABLE t1;
+EXECUTE IMMEDIATE 'SHOW DATABASES WHERE ?' USING DEFAULT;
+ERROR HY000: Default/ignore value is not supported for such parameter usage
+EXECUTE IMMEDIATE 'SHOW DATABASES WHERE ?' USING 0;
+Database
+#
 # End of 10.2 tests
 #

mysql-test/t/ps.test

@@ -4885,6 +4885,23 @@ drop procedure p;
 drop view v1;
 drop table t1;
 
+
+--echo #
+--echo # MDEV-22591 Debug build crashes on EXECUTE IMMEDIATE '... WHERE ?' USING IGNORE
+--echo #
+
+CREATE TABLE t1 (a INT);
+--error ER_INVALID_DEFAULT_PARAM
+EXECUTE IMMEDIATE 'SELECT * FROM t1 WHERE ?' USING IGNORE;
+--error ER_INVALID_DEFAULT_PARAM
+EXECUTE IMMEDIATE 'SELECT * FROM t1 HAVING ?' USING IGNORE;
+EXECUTE IMMEDIATE 'SELECT * FROM t1 WHERE ?' USING 0;
+EXECUTE IMMEDIATE 'SELECT * FROM t1 HAVING ?' USING 0;
+DROP TABLE t1;
+--error ER_INVALID_DEFAULT_PARAM
+EXECUTE IMMEDIATE 'SHOW DATABASES WHERE ?' USING DEFAULT;
+EXECUTE IMMEDIATE 'SHOW DATABASES WHERE ?' USING 0;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/item.h

@@ -3140,7 +3140,6 @@ class Item_param :public Item_basic_value,
 
   enum Type type() const
   {
-    DBUG_ASSERT(fixed || state == NO_VALUE);
     return item_type;
   }