trie: fix delete bug for values contained in fullNode

Delete crashed if a fullNode contained a valueNode directly. This bug is
very unlikely to occur with SecureTrie, but can happen with regular
tries. This commit also introduces a randomised test which triggers all
trie operations, which should prevent such bugs in the future.

Credit for finding this bug goes to Github user @rjl493456442.

trie/trie.go

@@ -377,6 +377,9 @@ func (t *Trie) delete(n node, prefix, key []byte) (bool, node, error) {
 		// n still contains at least two values and cannot be reduced.
 		return true, n, nil
 
+	case valueNode:
+		return true, nil, nil
+
 	case nil:
 		return false, nil, nil