Handle case where ip_address() returns NULL

ip_address() shouldn't return NULL, but if it does, this module will throw an exception, which it needn't do

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'ip' cannot be null: INSERT INTO {uiplog} (ip, uid, timestamp) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2); Array ( [:db_insert_placeholder_0] => [:db_insert_placeholder_1] => 1 [:db_insert_placeholder_2] => 1379461802 ) in uiplog_user_login() (line 28 of /Users/chris/Sites/example.org/sites/all/modules/uiplog/uiplog.module).