Fix handling of NULL char

This character is special because we cannot directly address it. We need to make
sure we are at the leaf node before.

tree.c

@@ -159,7 +159,7 @@ static int get_rev_path(struct tree *parent, unsigned char c, int *buf,
 	if (parent == NULL)
 		return -1;
 
-	if (parent->ch == c)
+	if (parent->ch == c && parent->left == NULL && parent->right == NULL)
 		return index;
 
 	if ((ret = get_rev_path(parent->left, c, buf, index)) >= 0)