Fix a UBsan bot.

If we do not initialize Prefix here, Prefix.data() returns a nullptr.
Later, it is passed to memcpy. memcpy's behavior is undefined if src (or
dst) is a nullptr even if a given size is 0. That's why this code
triggered UBsan.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314368 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed