Fixed problem with non-existing user not being detected.

Created by: ghaering

The effect I saw in production was that a "random" user was being shown in the activity log instead of the correct one. I think I found the reason for this. The change was not tested, and I do not really know Ruby but I hope I got the change from member to local variable right.

"@user" being a member variable, there normally was a @user object active, and ||= would keep it around if identify returns nil. Changing @user to a local variable fixes that and now non-existing users will actually be detected.