Dereference value-initialized map/set iterator in test_executors addTemporaryNode test (windows debug)

[brawner]

I believe this failure is different from other windows debug test_executor failures. @ivanpauno pointed out after resolving the previously existing issue in executors/graph_listener in #1404 (fixes #1282, #1340, #1391) that windows debug job flagged, this error also existed.

As with most windows debug failures, it's generally finding issues that exist on other platforms, but windows debug can be more pedantic sometimes.

Example failure:
https://ci.ros2.org/view/nightly/job/nightly_win_deb/1806/testReport/junit/projectroot.test/rclcpp/test_executors/