fix the bug in MemPostingReader::decodeNext(), which is similar to the bug in RTDiskPostingReader::decodeNext() in previous commit.
fix the bug in MemPostingReader::decodeNext(), which is similar to the
bug in RTDiskPostingReader::decodeNext() in previous commit.

pombredanne/izenelib,izenecloud/izenelib,izenecloud/izenelib,izenecloud/izenelib,pombredanne/izenelib,pombredanne/izenelib,izenecloud/izenelib,pombredanne/izenelib