We now null out smart pointers *before* calling unref on the pointee.
This ensures that the same smart pointer can't be used to acquire a new
reference to the pointee after its destruction has begun.
I ran into this when destroying a non-empty IntrusiveList of RefPtrs,
but the problem was more general so this fixes it for all of RefPtr,
NonnullRefPtr, OwnPtr and NonnullOwnPtr.
The insert_before method on AK::InlineLinkedList is used, so in order to
achieve feature parity, we need to implement it for AK::IntrusiveList as
well.
