functional tests fail --with-sanitizers=undefined #14059

Steps to reproduce on master and 0.17:

git checkout bitcoin/master && make distclean
./configure --with-sanitizers=undefined CC=clang CXX=clang++
make -j 16 check && ./test/functional/test_runner.py -j 16

Runs into some divide by zero and other issues...

This is addressed in the following PRs:

• #14242: Avoid triggering undefined behaviour (std::memset(nullptr, 0, 0)) if an invalid string is passed to DecodeSecret(...)
• #14239: Avoid dividing by zero (undefined behaviour) in EstimateMedianVal (policy)/ConnectTip (validation)/CreateTransaction (wallet)
• #13546: wallet: Avoid potential use of uninitialized value bnb_used in CWallet::CreateTransaction(...)
• #14252: build: Run functional tests and benchmarks under the undefined behaviour sanitizer (UBSan)

Please review :-)