Null parsing accepts bad input

[mtdowling]

Various places in this library that parse null will see 'n' and then skip the next three characters without validating that the next three characters are 'u' 'l' 'l'. So this is parsed as null: nope.

See for example https://github.com/json-iterator/java/blob/master/src/main/java/com/jsoniter/IterImplObject.java#L10-L12

    public static final String readObject(JsonIterator iter) throws IOException {
        byte c = IterImpl.nextToken(iter);
        switch (c) {
            case 'n':
                IterImpl.skipFixedBytes(iter, 3);
                return null;