[clangd] Fix an assertion in TypoCorrection.

Summary: https://github.com/clangd/clangd/issues/7

Reviewers: sammccall, hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57944

llvm-svn: 353514