Fix/trie ref long node key and empty is shared

[imlvts]

Fix TrieRef lookups with a node key longer than its buffer
new_with_key_and_path_in copied the focus's node key into a fixed stack
buffer, overflowing it (UB in release) when the key was longer, and
truncated key + path when the two together didn't fit. No child key is
longer than a node key, so only the first MAX_NODE_KEY_BYTES bytes of the
combined key can decide a step: look up that prefix, and keep stepping
through a child that begins within the node key. A combined key too long
to remain at a node is invalid.

(similar to Luke's 203b7d5))

Fix TrieRef::is_shared on an empty node