dict_stats_persistent_storage_check() may show garbage during --bootstrap

This came up while I was testing MDEV-14795 with some dummy initial database contents:

mkdir /dev/shm/data

echo "create database test;

use test;

create table t(a int,b char(255) not null default'')engine=innodb;

insert into t(a) select * from seq_1_to_1000000;

drop table t;"|

./mariadbd --datadir /dev/shm/data --bootstrap

The server error log output would include two garbled messages, both issued by dict_stats_persistent_storage_check(). The problem is that the local variable errmsg would remain uninitialized if opt_bootstrap holds. In this case, we should refrain from outputting the garbage message.

The code had been refactored between 10.5 and 10.6, so it is possible that this does not affect older versions than 10.6. The check for bootstrap had been added in MDEV-30065.