[clang-format] fix crash in NamespaceEndCommentsFixer (PR32438)

Summary:
The new test case was crashing before. Now it passes
as expected.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D31441

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299465 91177308-0d34-0410-b5e6-96231b3b80d8