dt_dev_pixelpipe_process_rec(): avoid integer overflow on big images
Initially reported by frogs (@IRC). Thanks!

Fixes a crash on 135MP+ images when zooming in (100%) on bottom right corner of image.

TODO: find a way to automatically check ALL the similar code and make sure that all of it handles this situation correctly.
(cherry picked from commit 177b49e3e639cf25eeda85b09cf6ce9321044167)