Interface conversion bug on pop()

[rtheunissen]

While doing some benchmarks, I ran into the following:

panic: interface conversion: vector.Value is *vector.nullSentinel, not *vector.Node

In context:

// Root node with only one child
for into.Shift > 0 && length < (1<<(into.Shift+BITS)) {
	into = into.Elements[0].(*Node)
}

[rtheunissen]

This is in the Truncate method.