Some more fixes in sequence functions

Initialize dictpos to 0, if it's a dict
Return -1 if PyDict_Next returns < 1

Change-Id: Ied5af0048b02a4a397b9f70679658f9f4c256a1b
Reviewed-on: http://review.couchbase.org/26327
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>