Polixenes is worried that in his absence, ills may be brewing in Bohemia. Superstitiously fearful that those very worries might cause bad things, he adds a wish that his fears are ill-founded. He ends by saying that if not, he will have to admit they were well-founded. The passage means something like:
I am worried that bad things could happen in Bohemia while I'm away. I hope that my worrying about those bad things does not make them manifest in real life. I do not want to have to say that my fears portrayed the truth!
He starts by stating his worries:
I am question'd by my feares, of what may chance,
Or breed upon our absence
He then hopes that these worries themselves do not jinx the situation in his country. Sneaping means bitterly and destructively cold. Merriam-Webster has:
sneaped; sneaping; sneaps
archaic : to blast or blight with cold : nip
That is to say, Polixenes's meta-worry is that his anxiety might itself act like a cold wind, blowing back to Bohemia to bring about the destruction he imagines. The antecedent of that is feares:
... that may blow
no sneaping Winds at home
And if his fears do bring about that destruction, then he will be forced to admit they were correct; that what they brought forth in his mind was in fact the truth . What is put forth is his imaginings, which turned out to be true:
... to make vs say,
This is put forth too truly.