Dtrie.Get() panics on unexisting key

[asido]

// Get returns the value for the associated key or returns nil if the
// key does not exist.
func (d *Dtrie) Get(key interface{}) interface{} {
	return get(d.root, d.hasher(key), key).Value()
}

get() returns nil if the key is not found and thus Value() raises a panic. Is that by design?

[AliceBennett]

#207