Fix a bug in manipulating the variable innodb_old_blocks_pct:

for any value assigned it got that value -1, except for 75. When
assigned 75, it got 75.

  mysql> set global innodb_old_blocks_pct=15;
  Query OK, 0 rows affected (0.00 sec)
  
  mysql> show variables like 'innodb_old_blocks_pct';
  +-----------------------+-------+
  | Variable_name         | Value |
  +-----------------------+-------+
  | innodb_old_blocks_pct | 14    | 
  +-----------------------+-------+
  1 row in set (0.00 sec)
  
  mysql> set global innodb_old_blocks_pct=75;
  Query OK, 0 rows affected (0.00 sec)
  
  mysql> show variables like 'innodb_old_blocks_pct';
  +-----------------------+-------+
  | Variable_name         | Value |
  +-----------------------+-------+
  | innodb_old_blocks_pct | 75    | 
  +-----------------------+-------+

After the fix it gets exactly what was assigned.

Approved by:	Marko (via IM)