trie: cleaner logic, one less func call

[karalabe]

This PR is a very tiny fix, that doesn't really have any impact other than code clarity. When hashing the account trie, whenever we reached a leaf node (account), we called onleaf. This onleaf method is used by statedb for trie pruning to link this leaf account to its storage trie.

There was a slight oversight I didn't realize while adding this logic: full trie nodes are often incomplete (i.e. some of its children are nil), but not flat out nil, rather valuenode(nil). This means that if child, ok := n.Children[i].(valueNode); ok { actually returns true and we call onleaf for dangling terminators. This is not an issue in our code, since onleaf tries to parse the leaf as an account, nil will fail parsing and returns, but this is an extra function call overhead we don't need.

Code clarity wise it took me 3 hours of debugging to realize why the path leading to such a "leaf" was invalid. Because it was not an actual leaf, just a dangling terminator. This is mostly what prompted this PR, so the next person touching this code won't waste 3 more hours ;P