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.
