SHOW GRANTS FOR <current_user> fails

If <user> does not have SELECT privilege on the `mysql` database and <user> is the current user the "SHOW GRANTS FOR <user>" (without hostname specified) statement fails with:

1044: Access denied for user '<user>'@'%' to database 'mysql'

This can be reproduced with the following test:

CREATE USER test;

connect (conn_1, localhost, test,,);

SHOW GRANTS FOR test;

This worked in 10.0.12, and seems to have been broken as part of the fix for MDEV-6625.