MDEV-29678 Valgrind/MSAN uninitialised value errors upon PS with ALTER under ONLY_FULL_GROUP_BY

st_select_lex::init_query is called in the exectuion of EXECUTE
IMMEDIATE 'alter table ...'. so reset the initialization at the
same point we set join= 0.

mysql-test/main/func_group.result

@@ -2548,5 +2548,9 @@ Warning	1292	Truncated incorrect DOUBLE value: 'x'
 Warning	1292	Truncated incorrect DOUBLE value: 'x'
 DROP TABLE t1;
 #
+#  MDEV-29678 Valgrind/MSAN uninitialised value errors upon PS with ALTER under ONLY_FULL_GROUP_BY
+#
+SET STATEMENT sql_mode=ONLY_FULL_GROUP_BY FOR EXECUTE IMMEDIATE 'ALTER TABLE mysql.time_zone_transition ORDER BY Time_zone_id, Transition_time';
+#
 # End of 10.3 tests
 #

mysql-test/main/func_group.test

@@ -1783,6 +1783,12 @@ CREATE TABLE t1 (a BIGINT) AS SELECT 1 AS v3 UNION SELECT FALSE ;
 SELECT DISTINCT a IN ( COLLATION (AVG ('x'))) FROM t1 ;
 DROP TABLE t1;
 
+--echo #
+--echo #  MDEV-29678 Valgrind/MSAN uninitialised value errors upon PS with ALTER under ONLY_FULL_GROUP_BY
+--echo #
+
+SET STATEMENT sql_mode=ONLY_FULL_GROUP_BY FOR EXECUTE IMMEDIATE 'ALTER TABLE mysql.time_zone_transition ORDER BY Time_zone_id, Transition_time';
+
 --echo #
 --echo # End of 10.3 tests
 --echo #

sql/sql_lex.cc

@@ -2321,6 +2321,7 @@ void st_select_lex::init_query()
   item_list.empty();
   min_max_opt_list.empty();
   join= 0;
+  cur_pos_in_select_list= UNDEF_POS;
   having= prep_having= where= prep_where= 0;
   cond_pushed_into_where= cond_pushed_into_having= 0;
   olap= UNSPECIFIED_OLAP_TYPE;