BitVector: Fix undefined behaviour

Calling reset() on an empty BitVector would call memset with a nullptr
argument which is undefined behaviour.

This should fix the sanitizer bot.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292575 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed