Bug #16297 In memory grant tables not flushed when users's hostname is ""

Description:
create user mysqltest_7@;
set password for mysqltest_7@ = password('systpass');
show grants for mysqltest_7@;
drop user mysqltest_7@;

# This "show grants" should return error saying that user does not exists, but unfortunately user still exists in memory grant tables
show grants for mysqltest_7@;

The information about database-level privileges is alose kept in memory.

How to repeat:
See above

Suggested fix:
Remove the user from the in memory grant structs when dropped.

Workaround: 
 - Call "flush privileges" after user has been dropped.

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/2127

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/2130

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/6760

Pushed to 5.0-maint

Pushed to 5.0.23 and 5.1.12, fixes and testcases for creating and manipulating users and grant when the user is something like  mysqltest_8@'' or  mysqltest_8

Noted in 5.0.23, 5.1.12 changelogs.

Grant table modifications sometimes did not refresh the in-memory
tables if the hostname was '' or not specified.