u.Hash - error

the u.Hash error can be safely ignored (panic) because multihash
only fails from the selection of hash function. If the fn + length
are valid, it won't error.

cc @whyrusleeping

merkledag.go

@@ -130,7 +130,7 @@ func (n *Node) Multihash() (mh.Multihash, error) {
 		return nil, err
 	}
 
-	return u.Hash(b)
+	return u.Hash(b), nil
 }
 
 // Key returns the Multihash as a key, for maps.