[clang] Don't segfault on incorrect using directive (PR41400)

Summary:
this is a bugfixe for [[ https://bugs.llvm.org/show_bug.cgi?id=41400 | PR41400 ]]

added nullptr check at the relevent place and test

Reviewers: rsmith, riccibruno

Reviewed By: rsmith

Subscribers: jkooker, jkorous, riccibruno, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60523

llvm-svn: 363360