diff: Fix integer overflows in Stone algorithm

Fix integer overflows that may occur when the context window is very
large and add tests to exercise those conditions.

Approved by: re (cperciva)
PR: 267032
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: thj, kevans
Differential Revision: https://reviews.freebsd.org/D55110

(cherry picked from commit 5fc739eb5949620da911db2f87ca8faedc549d3a)
(cherry picked from commit 237fd8dd96979412dd65ce95032aa9957463d069)