Fix an undefined variable error on Subs-Auth.php
If you failed a login attempt, SMF tries to log to the error log describing the failure. This error caused SMF to not log the IP associated to said user and PHP to throw an undefined variable error. This commit fixes both.

Signed-off-by: Yoshi2889 <rick.2889@gmail.com>
Merge pull request #2258 from Yoshi2889/auth-fix

Fix an undefined variable error on Subs-Auth.php