MDEV-16812 Semisync slave io thread segfaults at STOP-SLAVE handling

When the semisync slave is being stopped with STOP SLAVE just after
the master was shut down it attempts to reconnect with the master
anyway per a semisync routine.  Instead of an expected error the
io-thread segfauls in mysql_real_connect() execution at

 !mysql->options.extension->async_context

check trying to reach the extension's member while mysql->options.extension is
actually and correctly NULL.
Apparently not-NULL check for mysql->options.extension was missed and
it's deployed by the patch to fix this issue.

As a bonus it also tackles an assert
   Thread 0x7f16c72148c0 (LWP 24639) 0x00007f16c53b3bf2 in __GI___assert_fail (assertion=0x55a686117558 "global_status_var.global_memory_used == 0", file=0x55a6861171e8 "/home/andrei/MDB/WTs/10.3-clean/sql/mysqld.cc", line=2201, function=0x55a68611fa80 <mysqld_exit(int)::__PRETTY_FUNCTION__> "void mysqld_exit(int)") at assert.c:101

in a new test of the patch. The reason of the assert was insufficient cleanup
in Repl_semi_sync_slave::kill_connection() which has a branch where a MYSQL instance
was left out unfred.

mysql-test/suite/rpl/t/rpl_semi_sync_master_shutdown.test

@@ -0,0 +1,60 @@
+# MDEV-16812 Semisync slave io thread segfaults at STOP-SLAVE handling
+#
+# The test verifies that the semisync-enabled slave io thread
+# finishes off as specified in particular trying to connect even to a shut down
+# master for a semisync firewell routine.
+
+source include/not_embedded.inc;
+source include/have_debug.inc;
+source include/master-slave.inc;
+
+--connection master
+
+--let $sav_enabled_master=`SELECT @@GLOBAL.rpl_semi_sync_master_enabled `
+SET @@GLOBAL.rpl_semi_sync_master_enabled = 1;
+
+--connection slave
+source include/stop_slave.inc;
+--let $sav_enabled_slave=`SELECT @@GLOBAL.rpl_semi_sync_slave_enabled `
+SET @@GLOBAL. rpl_semi_sync_slave_enabled = 1;
+source include/start_slave.inc;
+
+--connection master
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 SET a=1;
+
+--sync_slave_with_master
+
+connection master;
+--echo # Shutdown master
+--let $rpl_server_number=1
+source include/rpl_stop_server.inc;
+
+--connection slave
+--source include/stop_slave.inc
+
+#connection master;
+--echo # Restart master
+--let $rpl_server_number=1
+source include/rpl_start_server.inc;
+
+#
+# Clean up
+#
+--connection slave
+--source include/stop_slave.inc
+--source include/start_slave.inc
+
+--connection master
+SET @@GLOBAL.debug_dbug="";
+--eval SET @@GLOBAL. rpl_semi_sync_master_enabled = $sav_enabled_master
+
+--connection master
+DROP TABLE t1;
+
+--sync_slave_with_master
+source include/stop_slave.inc;
+--eval SET @@GLOBAL. rpl_semi_sync_slave_enabled = $sav_enabled_slave
+
+--let $rpl_only_running_threads= 1
+--source include/rpl_end.inc