Use IEEE infinity, not 1e10, for null-and-not-null case in gistpenalty().

Use IEEE infinity, not 1e10, for null-and-not-null case in gistpenalty().

Use of a randomly chosen large value was never exactly graceful, and
now that there are penalty functions that are intentionally using infinity,
it doesn't seem like a good idea for null-vs-not-null to be using something
less.