workaround for bug #21797

at a certain point in playcampaign.cpp after the level has ended, wesnoth was
grabbing .child("end_level_data"), and writing carry_over sides.
this would cause "mandatory child missing" errors in the test case described.

we fix the error by checking if the child was not present and if so adding it.
however, it may indicate a more fundamental bug involving reloaded games.