Fix a case when the TemplateArgs vector can be empty.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@72911 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/SemaTemplate/default-arguments.cpp b/test/SemaTemplate/default-arguments.cpp
index 572227c..0e4c366 100644
--- a/test/SemaTemplate/default-arguments.cpp
+++ b/test/SemaTemplate/default-arguments.cpp
@@ -10,3 +10,6 @@
 template<typename U = float, int M> struct X;
 
 X<> *x4;
+
+template<typename T = int> struct Z;
+template struct Z<>;