Fix prime generation loop in crypto/bn/bn_prime.pl by making

Fix prime generation loop in crypto/bn/bn_prime.pl by making
sure the loop does correctly stop and breaking ("division by zero")
modulus operations are not performed. The (pre-generated) prime
table crypto/bn/bn_prime.h was already correct, but it could not be
re-generated on some platforms because of the "division by zero"
situation in the script.