Every path in a radix trie ends with a leaf or a NULL. By replacing
NULL (non-leaf) pointers with NULL leaves, there is a NULL test
removed from every iteration of an index-based search loop.

This speeds up radix trie searches by few percent. If there are any
radix tries that are not initialized with the init() function, but
instead depend on zeroing everything being proper initialization, this
will break those tries.

Reviewed by: alc, kib
Tested by: pho (as part of a larger change)
Differential Revision: https://reviews.freebsd.org/D41171