event: fix crash on child-source state modifications

Setting a child-source state to anything but SD_EVENT_OFF currently does
nothing. The condition logic is flawed. Move the state update *below* the
test for the previous state.

Fixes a crash if you call:
  sd_event_source_set_enabled(source, SD_EVENT_ON);
  sd_event_source_unref(source);
on a child-source in its own callback.