Fix build error about variable 'third' is uninitialized when used here

[luyahan]

toolchain version

luyahan@plct-c7:~/source/highway$ clang -v
clang version 17.0.2 (https://github.com/llvm/llvm-project.git b2417f51dbbd7435eb3aaf203de24de6754da50e)
Target: riscv64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/luyahan/toolchain/llvm/bin
Found candidate GCC installation: /home/luyahan/toolchain/llvm/bin/../lib/gcc/riscv64-unknown-linux-gnu/13.2.0
Selected GCC installation: /home/luyahan/toolchain/llvm/bin/../lib/gcc/riscv64-unknown-linux-gnu/13.2.0

[jan-wassenberg]

Currently we have a conflict. Please rebase and include only the new commit :)