mongo-wire: Fix mongo_wire_reply_packet_get_nth_document().
mongo-wire: Fix mongo_wire_reply_packet_get_nth_document().

Much like the bson problem, mongo_wire_reply_packet_get_nth_document()
was suffering from a casting mistake, and only read the first byte of
the document length.

Fixed it the same way, to read all of it.

algernon/libmongo-client,algernon/libmongo-client,algernon/libmongo-client