The SQL-mode PAD_CHAR_TO_FULL_LENGTH could prevent a DROP USER
statement from privileges associated with the user being dropped.
What ocurred was that reading from the User and Host fields of
the tables tables_priv or columns_priv would yield values padded
with spaces, causing a failure to match a specified user or host 
('user' != 'user     ');

The solution is to disregard the PAD_CHAR_TO_FULL_LENGTH mode
when iterating over and matching values in the privileges tables
for a DROP USER statement.

mysql-test/r/sql_mode.result:
  Add test case result for Bug#45100.
mysql-test/t/sql_mode.test:
  Add test case for Bug#45100.
sql/sql_acl.cc:
  Clear MODE_PAD_CHAR_TO_FULL_LENGTH before dropping privileges.