The issue was that the `user_data` pointer in the local user data
is not necessarily valid, because a different user-data might be
passed in when a lazy-function is called a second time (it's semantically
the same user-data but, might have a different pointer).
The solution is to not keep around the dangling pointer but to pass
it in when it is actually needed. I still keep the old constructor of the
local-user-data, because more stuff might be added to it, that needs
the user-data, as was the case in the past already (before 142541c27).
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