Did the bees die with thier heads in the cells? That is an almost sure sign of starvation. It would be unusuall for a hive to die of AFB in the winter, although, if they were weakend over the summer and fall, that is a different story. Did you ever notice greasy cappings with multiple holes in the brood nest in the summer or fall when the bees were still alive? I'd have to say that there is a very...very small chance of your bees getting any possible disease from these used frames. Probably the biggest risk is a few Varroa related viruses which can actually survive in the comb but, just about every hive (every hive I've been in) has this so, it's of minor concern.