MDEV-4712 : Fix "shutdown" test.

There is a race condition during shutdown, that happens when "show status" runs during shutdown. 
Functions show_slave_running(), show_slave_received_heartbeats(), show_heartbeat_period() would dereference NULL pointer master_info_index, after this variable is  set to NULL in slave_end() during shutdown.
Fix introduces check for master_info_index == NULL in the affected functions.

mysql-test/t/shutdown.test

@@ -11,13 +11,14 @@ shutdown;
 
 connection default;
 disconnect c1;
+--connect (c1,localhost,root,,)
 
 --let $_server_id= `SELECT @@server_id`
 --let $_expect_file_name= $MYSQLTEST_VARDIR/tmp/mysqld.$_server_id.expect
 --exec echo "wait" > $_expect_file_name
 
-shutdown;
-
+--send shutdown
+--connection default
 --source include/wait_until_disconnected.inc
 
 --exec echo "restart" > $_expect_file_name

sql/mysqld.cc

@@ -6993,7 +6993,7 @@ static int show_rpl_status(THD *thd, SHOW_VAR *var, char *buff)
 
 static int show_slave_running(THD *thd, SHOW_VAR *var, char *buff)
 {
-  Master_info *mi;
+  Master_info *mi= NULL;
   bool tmp;
   LINT_INIT(tmp);
 
@@ -7001,12 +7001,15 @@ static int show_slave_running(THD *thd, SHOW_VAR *var, char *buff)
   var->value= buff;
   mysql_mutex_unlock(&LOCK_status);
   mysql_mutex_lock(&LOCK_active_mi);
+  if (master_info_index) 
+  {
     mi= master_info_index->
       get_master_info(&thd->variables.default_master_connection,
                       Sql_condition::WARN_LEVEL_NOTE);
     if (mi)
       tmp= (my_bool) (mi->slave_running == MYSQL_SLAVE_RUN_CONNECT &&
                       mi->rli.slave_running);
+  }
   mysql_mutex_unlock(&LOCK_active_mi);
   mysql_mutex_lock(&LOCK_status);
   if (mi)
@@ -7019,7 +7022,7 @@ static int show_slave_running(THD *thd, SHOW_VAR *var, char *buff)
 
 static int show_slave_received_heartbeats(THD *thd, SHOW_VAR *var, char *buff)
 {
-  Master_info *mi;
+  Master_info *mi= NULL;
   longlong tmp;
   LINT_INIT(tmp);
 
@@ -7027,11 +7030,14 @@ static int show_slave_received_heartbeats(THD *thd, SHOW_VAR *var, char *buff)
   var->value= buff;
   mysql_mutex_unlock(&LOCK_status);
   mysql_mutex_lock(&LOCK_active_mi);
+  if (master_info_index) 
+  {
     mi= master_info_index->
       get_master_info(&thd->variables.default_master_connection,
                       Sql_condition::WARN_LEVEL_NOTE);
     if (mi)
       tmp= mi->received_heartbeats;
+  }
   mysql_mutex_unlock(&LOCK_active_mi);
   mysql_mutex_lock(&LOCK_status);
   if (mi)
@@ -7044,7 +7050,7 @@ static int show_slave_received_heartbeats(THD *thd, SHOW_VAR *var, char *buff)
 
 static int show_heartbeat_period(THD *thd, SHOW_VAR *var, char *buff)
 {
-  Master_info *mi;
+  Master_info *mi= NULL;
   float tmp;
   LINT_INIT(tmp);
 
@@ -7052,11 +7058,14 @@ static int show_heartbeat_period(THD *thd, SHOW_VAR *var, char *buff)
   var->value= buff;
   mysql_mutex_unlock(&LOCK_status);
   mysql_mutex_lock(&LOCK_active_mi);
+  if (master_info_index) 
+  {
     mi= master_info_index->
       get_master_info(&thd->variables.default_master_connection,
                     Sql_condition::WARN_LEVEL_NOTE);
     if (mi)
       tmp= mi->heartbeat_period;
+  }
   mysql_mutex_unlock(&LOCK_active_mi);
   mysql_mutex_lock(&LOCK_status);
   if (mi)