MDEV-30408 Reset explicit_limit in exists2in

Item_exists_subselect::fix_length_and_dec() sets explicit_limit to 1.
In the exists2in transformation it resets select_limit to NULL. For
consistency we should reset explicity_limit too.

This fixes a bug where spider table returns wrong results for queries
that gets through exists2in transformation when semijoin is off.

sql/item_subselect.cc

@@ -3186,6 +3186,7 @@ bool Item_exists_subselect::exists2in_processor(void *opt_arg)
   set_exists_transformed();
 
   first_select->select_limit= NULL;
+  first_select->explicit_limit= FALSE;
   if (!(in_subs= new (thd->mem_root) Item_in_subselect(thd, left_exp,
                                                          first_select)))
   {

storage/spider/mysql-test/spider/bugfix/r/mdev_30408.result

+for master_1
+for child2
+for child3
+set @@optimizer_switch="semijoin=off";
+set spider_same_server_link= 1;
+CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+create table ten(a int primary key);
+insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t2 (a int, b int);
+insert into t2 select a,a from ten;
+create table t1 (a int, b int) ENGINE=Spider
+COMMENT='WRAPPER "mysql", srv "srv",TABLE "t2"';
+select a.a, a.b from t1 a where exists (select * from t1 b where b.b = a.b);
+a	b
+0	0
+1	1
+2	2
+3	3
+4	4
+5	5
+6	6
+7	7
+8	8
+9	9
+drop table ten, t1, t2;
+drop server srv;
+for master_1
+for child2
+for child3

storage/spider/mysql-test/spider/bugfix/t/mdev_30408.test

+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+
+set @@optimizer_switch="semijoin=off";
+set spider_same_server_link= 1;
+evalp CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+
+create table ten(a int primary key);
+insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t2 (a int, b int);
+insert into t2 select a,a from ten;
+create table t1 (a int, b int) ENGINE=Spider
+COMMENT='WRAPPER "mysql", srv "srv",TABLE "t2"';
+select a.a, a.b from t1 a where exists (select * from t1 b where b.b = a.b);
+drop table ten, t1, t2;
+drop server srv;
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log