set active context to nullptr in ~scheduler()

src/context.cpp

@@ -44,6 +44,11 @@ context::active( context * active) noexcept {
     active_ = active;
 }
 
+void
+context::reset_active() noexcept {
+    active_ = nullptr;
+}
+
 void
 context::set_terminated_() noexcept {
     // protect for concurrent access