math/strongprime.c: Clamp the starting point.

math/strongprime.c: Clamp the starting point.

Now the result will be in the upper quarter of the `obvious' range, and
the product of two such values is guaranteed to have the desired number
of bits.  This saves callers from doing stupid things like trying to
clamp one of the factors by hand, which ends up significantly biasing
the second factor.  (This isn't very bad, because there's a /lot/ of
randomness in the chosen congruence class, but it's good to fix this
sort of thing.)