Might you forget there's a few people inside winterfell and there's catacombs under winterfell with undoubtedly an exit outside it's walls. Stannis will get help from Reek, go through the catacombs at night, claim the lord of light favoured him thanks to his sacrifice and wins the North. OR draw the Boltons out as he captures the dreadfort instead. All in all i am pretty sure thanks to the lord of light, some help or good fortune will come.
After episode 8, i am pretty damn sure there is only 1 thing that can save westeros from winter and certain death...And that is Daenerys and her dragons.