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.

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);