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/r/rpl_semi_sync_master_shutdown.result

@@ -0,0 +1,33 @@
+include/master-slave.inc
+[connection master]
+connection master;
+SET @@GLOBAL.rpl_semi_sync_master_enabled = 1;
+connection slave;
+include/stop_slave.inc
+SET @@GLOBAL. rpl_semi_sync_slave_enabled = 1;
+include/start_slave.inc
+connection master;
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 SET a=1;
+connection slave;
+connection master;
+# Shutdown master
+include/rpl_stop_server.inc [server_number=1]
+connection slave;
+include/stop_slave.inc
+# Restart master
+include/rpl_start_server.inc [server_number=1]
+connection slave;
+include/stop_slave.inc
+Warnings:
+Note	1255	Slave already has been stopped
+include/start_slave.inc
+connection master;
+SET @@GLOBAL.debug_dbug="";
+SET @@GLOBAL. rpl_semi_sync_master_enabled = 0;
+connection master;
+DROP TABLE t1;
+connection slave;
+include/stop_slave.inc
+SET @@GLOBAL. rpl_semi_sync_slave_enabled = 0;
+include/rpl_end.inc

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

sql-common/client.c

@@ -3657,7 +3657,7 @@ error:
     end_server(mysql);
     mysql_close_free(mysql);
     if (!(client_flag & CLIENT_REMEMBER_OPTIONS) &&
-        !mysql->options.extension->async_context)
+        !(mysql->options.extension && mysql->options.extension->async_context))
       mysql_close_free_options(mysql);
   }
   DBUG_RETURN(0);

sql/semisync_slave.cc

@@ -144,8 +144,7 @@ void Repl_semi_sync_slave::kill_connection(MYSQL *mysql)
   {
     sql_print_information("cannot connect to master to kill slave io_thread's "
                           "connection");
-    if (!ret)
-      mysql_close(kill_mysql);
+    mysql_close(kill_mysql);
     return;
   }
   size_t kill_buffer_length = my_snprintf(kill_buffer, 30, "KILL %lu",