sieve: fix overrun bug in Sieve::primes_from.
sieve: fix overrun bug in Sieve::primes_from.

It was considering some things at the tail end to be primes.

Antiarchitect/primal,Antiarchitect/primal,wackywendell/primal,cuviper/primal,huonw/primal,huonw/primal,cuviper/primal,wackywendell/primal