[YAML] Fix undefined behavior.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@172338 91177308-0d34-0410-b5e6-96231b3b80d8
[test] Fix tests on Windows.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@172339 91177308-0d34-0410-b5e6-96231b3b80d8