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

llvm-svn: 243996