Mark LWG issue 2276 as complete. Add _LIBCPP_ASSERTS for it

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@271247 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/future b/include/future
index 957a23c..3bbb925 100644
--- a/include/future
+++ b/include/future
@@ -1514,6 +1514,7 @@
 void
 promise<_Rp>::set_exception_at_thread_exit(exception_ptr __p)
 {
+    _LIBCPP_ASSERT( __p != nullptr, "promise::set_exception_at_thread_exit: received nullptr" );
     if (__state_ == nullptr)
         __throw_future_error(future_errc::no_state);
     __state_->set_exception_at_thread_exit(__p);
@@ -1649,6 +1650,7 @@
 void
 promise<_Rp&>::set_exception_at_thread_exit(exception_ptr __p)
 {
+    _LIBCPP_ASSERT( __p != nullptr, "promise::set_exception_at_thread_exit: received nullptr" );
     if (__state_ == nullptr)
         __throw_future_error(future_errc::no_state);
     __state_->set_exception_at_thread_exit(__p);