Don't create a null reference to NestedNameSpecifier.

This bug was reported by UBSan.

llvm-svn: 216691
diff --git a/clang/lib/ASTMatchers/ASTMatchFinder.cpp b/clang/lib/ASTMatchers/ASTMatchFinder.cpp
index 050630f..6c5bc3c 100644
--- a/clang/lib/ASTMatchers/ASTMatchFinder.cpp
+++ b/clang/lib/ASTMatchers/ASTMatchFinder.cpp
@@ -731,7 +731,8 @@
   match(NNS);
   // We only match the nested name specifier here (as opposed to traversing it)
   // because the traversal is already done in the parallel "Loc"-hierarchy.
-  match(*NNS.getNestedNameSpecifier());
+  if (NNS.hasQualifier())
+    match(*NNS.getNestedNameSpecifier());
   return
       RecursiveASTVisitor<MatchASTVisitor>::TraverseNestedNameSpecifierLoc(NNS);
 }