Make sure nesting arrays are compacted to avoid memory overflow

The PR will fix https://github.com/visionmedia/node-querystring/issues/104 in a way that is consistent with the top level array compaction.

Please note that https://github.com/visionmedia/node-querystring/issues/104 is a serious security concern as malicious input string such as `foo[0][100000000]` will crash the node process.