mysql_upgrade can delete all user

In mysql_upgrade the new table global_priv will be created like this:

CREATE TABLE IF NOT EXISTS global_priv  [...]

select  Host, User, JSON_COMPACT from mysql.user.

 DROP TABLE user; 

If this select fails, all user will be deleted in the migration process.

So DROP TABLE user; should only be executed,
if the select get no error or return at least 1 row.