Bug #26000 SHOW SLAVE STATUS can crash mysqld during shutdown process

active_mi has been reset (shutdown) at the time of quering with 
SHOW SLAVE STATUS so that 
at handling of SHOW an attempt to read its members segfaults.

Fixed with checking the value of active_mi before to call show_master_info()
Merely send_ok() is invoked when active_mi does not exist.
A test can not be easiely written.

Notice, there are more analogical cases in the code which require a similar
treatment (to be reported as a bug separately). 

sql/sql_parse.cc

@@ -2844,7 +2844,16 @@ mysql_execute_command(THD *thd)
     if (check_global_access(thd, SUPER_ACL | REPL_CLIENT_ACL))
       goto error;
     pthread_mutex_lock(&LOCK_active_mi);
-    res = show_master_info(thd,active_mi);
+    if (active_mi != NULL)
+    {
+      res = show_master_info(thd, active_mi);
+    }
+    else
+    {
+      push_warning(thd, MYSQL_ERROR::WARN_LEVEL_WARN, 0,
+                   "the master info structure does not exist");
+      send_ok(thd);
+    }
     pthread_mutex_unlock(&LOCK_active_mi);
     break;
   }