MDEV-8224 Server crashes in get_server_from_table_to_cache on empty name

mysql-test/r/empty_server_name-8224.result

+create server '' foreign data wrapper w2 options (host '127.0.0.1');

mysql-test/t/empty_server_name-8224.test

+create server '' foreign data wrapper w2 options (host '127.0.0.1');
+--exec echo "wait" > $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+--shutdown_server 10
+--source include/wait_until_disconnected.inc
+--exec echo "restart" > $MYSQLTEST_VARDIR/tmp/mysqld.1.expect

sql/sql_servers.cc

@@ -326,7 +326,8 @@ get_server_from_table_to_cache(TABLE *table)
   table->use_all_columns();
 
   /* get each field into the server struct ptr */
-  server->server_name= get_field(&mem, table->field[0]);
+  ptr= get_field(&mem, table->field[0]);
+  server->server_name= ptr ? ptr : blank;
   server->server_name_length= (uint) strlen(server->server_name);
   ptr= get_field(&mem, table->field[1]);
   server->host= ptr ? ptr : blank;