[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
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 2e5d1c7..b59f3fa 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -90,7 +90,7 @@
   // When naming a constructor as a member of a dependent context (eg, in a
   // friend declaration or an inherited constructor declaration), form an
   // unresolved "typename" type.
-  if (CurClass->isDependentContext() && !EnteringContext) {
+  if (CurClass->isDependentContext() && !EnteringContext && SS.getScopeRep()) {
     QualType T = Context.getDependentNameType(ETK_None, SS.getScopeRep(), &II);
     return ParsedType::make(T);
   }