- In function 'handle_grant_struct' when searching the memory structures for an 
   entry to modify, convert all entries here host.hostname is NULL to "" and compare that 
   with the host passed in argument "user_from".
 - A user created with hostname "" is stored in "mysql.user" table as host="" but when loaded into 
   memory it'll be stored as host.hostname NULL. Specifiying "" as hostname means
   that "any host" can connect. Thus is's correct to turn on allow_all_hosts
   when such a user is found. 
 - Review and fix other places where host.hostname may be NULL.