[clangd] Fix an assertion crash in "ExtractVariable" tweak

Summary:
GetTypePtr requires that the type should not be null, otherwise we hit
an assertion, we should use getTypePtrOrNull instead.

Reviewers: sammccall, SureYeaah

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64556

llvm-svn: 365763