settings: invalidate pointers for debugging use of outdated connections_cached_list
connections_cached_list stays only valid until we remove/add connections to NMSettings. Using the list without cloning requires to be aware of that. When clearing the list, invalidate all pointers, in the hope that a following use-after-free will blow up with an assertion. We only do this in elevated assertion mode. It's not to prevent any bugs, it's to better notice it.
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