[clang-tidy] Fix an assert failure in `modernize-loop-convert`.

Summary:
The test code will trigger following an assert failure:

assert.h assertion failed at LoopConvertUtils.cpp:560 in
bool clang::tidy::modernize::ForLoopIndexUseVisitor::TraverseMemberExpr(clang::MemberExpr*): ExprType->isPointerType() && "Operator-> returned non-pointer type"

Reviewers: alexfh

Differential Revision: http://reviews.llvm.org/D17287

llvm-svn: 260953