Don't drop core if the user passes a NULL pointer to WT_CURSOR.get_value()

@michaelcahill, this change is based on @agorrod  issue #579.

I went ahead and pushed the change to avoid dropping core if a statistics cursor's get-value argument is NULL, but I'm less confident this change is complete (and I wanted you to have a chance to Just Say No).

I couldn't think of any reason it's better to drop core, although it might be harder to debug the case where the application is unintentionally passing a NULL pointer to WT_CURSOR.get_value().