Fix bug 1963: Web UI throws warnings during previously open login session after user account is deleted
Fix bug 1963: Web UI throws warnings during previously open login session after user account is deleted

common_logged_in() returned bogus results because it checks against null specifically, but common_current_user() was sticking 'false' into $_cur because that's what User::staticGet() returned from a failed lookup. Now we skip over a failed lookup here, so we keep null and all is well.

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