fix for replication , because sub_iv may be NULL in extreme scenario

[xingyingone]

fix for replication , because sub_iv may be NULL in extreme scenario

[xingyingone]

for mysql5.7.2 may crash

(gdb) bt
#0 __pthread_kill (threadid=, signo=11) at ../sysdeps/unix/sysv/linux/pthread_kill.c:57
#1 0x000000000094ac1d in handle_fatal_signal (sig=11) at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/signal_handler.cc:220
#2
#3 Gtid_set::is_interval_subset (sub=sub@entry=0x7fe0a91ecbc0, super=super@entry=0x7fe0a91ecbd0) at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/rpl_gtid_set.cc:1149
#4 0x0000000000f63367 in Gtid_set::is_subset_for_sid (this=, super=super@entry=0x7fe0a91ecc10, superset_sidno=, subset_sidno=subset_sidno@entry=3)
at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/rpl_gtid_set.cc:1212
#5 0x0000000000fa54e3 in Binlog_sender::check_start_file (this=this@entry=0x7fe0a91ed760) at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/rpl_binlog_sender.cc:758
#6 0x0000000000fa5861 in Binlog_sender::init (this=this@entry=0x7fe0a91ed760) at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/rpl_binlog_sender.cc:103
#7 0x0000000000fa6a04 in Binlog_sender::run (this=this@entry=0x7fe0a91ed760) at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/rpl_binlog_sender.cc:218
#8 0x0000000000fa35c3 in mysql_binlog_send (thd=thd@entry=0x7fe0ade0b000, log_ident=log_ident@entry=0x7fe0a91ee320 "", pos=pos@entry=4, slave_gtid_executed=slave_gtid_executed@entry=0x7fe0a91ee130, flags=flags@entry=4)
at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/rpl_master.cc:412
#9 0x0000000000fa4044 in com_binlog_dump_gtid (thd=thd@entry=0x7fe0ade0b000, packet=, packet_length=) at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/rpl_master.cc:396
#10 0x0000000000dee7e4 in dispatch_command (thd=thd@entry=0x7fe0ade0b000, com_data=com_data@entry=0x7fe0a91eedb0, command=COM_BINLOG_DUMP_GTID) at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/sql_parse.cc:1699
#11 0x0000000000defaed in do_command (thd=thd@entry=0x7fe0ade0b000) at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/sql_parse.cc:1021
#12 0x0000000000ea2d74 in handle_connection (arg=arg@entry=0x7fe0b47a6310) at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/sql/conn_handler/connection_handler_per_thread.cc:303
#13 0x0000000001330e11 in pfs_spawn_thread (arg=0x7fe0a9c3e920) at /home/fengsen/data/mysql-5.7.20-v3e/code/mysql-5.7.20/storage/perfschema/pfs.cc:2190
#14 0x00007fe36c8374a4 in start_thread (arg=0x7fe0a91ef700) at pthread_create.c:456
#15 0x00007fe36b496d0f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97

[mysql-oca-bot]

Hi, thank you for your contribution. Please confirm this code is submitted under the terms of the OCA (Oracle's Contribution Agreement) you have previously signed by cutting and pasting the following text as a comment:
"I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it."
Thanks

[mysql-oca-bot]

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