[libcxx] Fix pop_back() tests to make sure they don't always just pass

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@349071 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/containers/sequences/deque/pop_back_empty.pass.cpp

@@ -22,4 +22,5 @@ int main() {
     q.push_back(0);
     q.pop_back();
     q.pop_back();
+    std::exit(1);
 }

test/libcxx/containers/sequences/vector/pop_back_empty.pass.cpp

@@ -22,4 +22,5 @@ int main() {
     v.push_back(0);
     v.pop_back();
     v.pop_back();
+    std::exit(1);
 }