Avoid passing nullptr to std::equal.

Avoid passing nullptr to std::equal.
As documented in the LLVM Coding Standards, indeed MSVC incorrectly asserts
on this in Debug mode. This happens when building clang with Visual C++ and
-triple i686-pc-windows-gnu on these clang regression tests:

 clang/test/CodeGen/2011-03-08-ZeroFieldUnionInitializer.c
 clang/test/CodeGen/empty-union-init.c

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243996 91177308-0d34-0410-b5e6-96231b3b80d8