HMACTest.HMAC_MD5_RFC2202_SIGN crashes under newer compilers

[ RUN      ] HMACTest.HMAC_MD5_RFC2202_SIGN
/usr/include/c++/12.1.0/bits/stl_vector.h:1123: std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](size_type) [with _Tp = unsigned char; _Alloc = std::allocator<unsigned char>; reference = unsigned char&; size_type = long unsigned int]: Assertion '__n < this->size()' failed.
FAIL: run_unittests

Can be replicated on Ubuntu 22.04.

Here is a job that fails: https://jenkins.aws.isc.org/job/kea-dev/job/ut-extended/772/testReport/junit/(root)/Crash/run_tests___ubuntu_22_04_amd64___ubuntu_22_04_amd64_results_____RUN________HMACTest_HMAC_MD5_RFC2202_SIGN/

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