Proactively reject registration of nil functions.
Proactively reject registration of nil functions.

Surely no one would try to do this, nor then be surprised if it
creates problems.  On the other hand: if someone *does* do this, and
the error doesn't appear until arbitrarily far away when the map is
read, it's a pain to diagnose... and checking it up front is cheap.
So, here we go: check it up front.

jbenet/go-multihash,jbenet/go-multihash