Fix internal next_prime(n) function.

Fix internal next_prime(n) function.

This helper function was doing wild things when called the wrong way.
E.g. it would allocate obscene amounts of memory and run for ages when
called with an argument that has never been returned by it before.

In the context where it's being used so far this is not a problem. But
it's better to make it foolproof and spare a new user some possible
despair and distress.