TROVE-2024-003: With lite vanguards, STUB circuits always have length 2

In !2145 (comment 3030149), we spotted a bug where STUB circuits were never extended past the L2 vanguard. The problem was that this condition does not have an else branch (unless full vanguards are in use, the circuit is returned after adding an L2 vanguard, which is incorrect).

EDIT: This is also tracked as CVE-2024-35313.