Fixed issue in SessionLoginThrottle.module

Reset the number of login attempts for a user if more than $requireSeconds have elapsed since his last login attempt. 
This fixes an issue than occurs if only one user logins to the  system during a certain period of time: in this case, the number of attempts for this user was always increased regardless of the time interval since the last login attempt. If the user entered a wrong password once, he was forced to wait the max time before retrying, even if his previous connection was several hours ago!

Note: In this case, the user entry in the table is never deleted since the SQL DELETE command is executed at the end. An alternative fix would be to delete the outdated rows in the table before doing the test for the curent user.