Missing value barrier in liboqs FrodoKEM implicit rejection path

liboqs is a C-language cryptographic library that provides implementations of post-quantum cryptography algorithms. Prior to 0.16.0, the ct_select function in src/kem/frodokem/external/util.c for the FrodoKEM-640, FrodoKEM-976, and FrodoKEM-1344 AES and SHAKE reference variants lacks a value barrier in the Fujisaki-Okamoto implicit-rejection path. When compiled with Clang 17 or later at an affected optimization level such as -Os, the compiler can emit a secret-dependent conditional load, causing the real and rejection secret paths to leave distinguishable cache residency after successful and failed decapsulation. A local attacker able to perform repeated cache-timing measurements may use this oracle as part of a private-key recovery attack. Default -O3 builds vectorize both source loads and do not exhibit the reported behavior, and the advisory does not report the same generated memory-access pattern in NTRU, NTRU-Prime, BIKE, HQC, Classic McEliece, Kyber, or the already-barriered ML-KEM implementation. This issue is fixed in version 0.16.0.