``pt-online-schema-change`` regression caused it to hang for a stopped replica when using replication channels on the slave.

Hi,

pt-online-schema-changes 3.0.13 hangs for a stopped replica when using replication channels on the slave and the slave status is fine.

How to reproduce:

– Deploy Master/Slave topology
$ dbdeployer deploy replication /opt/percona_server/5.7.26

– On the slave reset the slave and manually add a replication channel
slave1 [localhost:19328]

{msandbox} ((none)) > reset slave all ;
Query OK, 0 rows affected (0.01 sec)

slave1 [localhost:19328] {msandbox}

((none)) > CHANGE MASTER TO MASTER_HOST='127.0.0.1', MASTER_PORT=19327, MASTER_USER = 'msandbox', MASTER_PASSWORD='msandbox', MASTER_AUTO_POSITION=1 FOR CHANNEL 'channel1';
Query OK, 0 rows affected, 2 warnings (0.01 sec)

slave1 [localhost:19328]

{msandbox}

((none)) > START SLAVE ;
Query OK, 0 rows affected, 1 warning (0.00 sec)

– Run pt-online-schema-change 3.0.13 fails with error in line 4504
[PROD-SER] [juan.arruti@bm-support01 master]$ pt-online-schema-change --user=root --password=msandbox --host=127.0.0.1 --port=19327 --recurse 1 --max-lag 2 --check-slave-lag=F=/home/juan.arruti/sandboxes/rsandbox_5_7_26/node1/my.sandbox.cnf --execute --alter "force" D=test,t=t1
Found 1 slaves:
bm-support01.bm.int.percona.com -> 127.0.0.1:19328
Will check slave lag on:
bm-support01.bm.int.percona.com -> 127.0.0.1:19327
Use of uninitialized value in string ne at /usr/bin/pt-online-schema-change line 4504.

– If I add --no-check-replication-filters hangs indefinitely
[PROD-SER] [juan.arruti@bm-support01 master]$ pt-online-schema-change --user=root --password=msandbox --host=127.0.0.1 --port=19327 --recurse 1 --max-lag 2 --check-slave-lag=F=/home/juan.arruti/sandboxes/rsandbox_5_7_26/node1/my.sandbox.cnf --no-check-replication-filters --execute --alter "force" D=test,t=t1

Found 1 slaves:
bm-support01.bm.int.percona.com -> 127.0.0.1:19328
Will check slave lag on:
bm-support01.bm.int.percona.com -> 127.0.0.1:19327
Operation, tries, wait:
analyze_table, 10, 1
copy_rows, 10, 0.25
create_triggers, 10, 1
drop_triggers, 10, 1
swap_tables, 10, 1
update_foreign_keys, 10, 1
Altering `test`.`t1`...
Creating new table...
Created new table test._t1_new OK.
Altering new table...
Altered `test`.`_t1_new` OK.
2019-08-30T15:38:36 Creating triggers...
2019-08-30T15:38:36 Created triggers OK.
2019-08-30T15:38:36 Copying approximately 1 rows...
Replica bm-support01.bm.int.percona.com is stopped. Waiting.
Replica bm-support01.bm.int.percona.com is stopped. Waiting.
Replica bm-support01.bm.int.percona.com is stopped. Waiting.
Replica bm-support01.bm.int.percona.com is stopped. Waiting.
...