fix: prevent buffer jump when safe strategy fails

[AjaiKN]

When replace-buffer-contents fails to do a non-destructive replacement within the allotted time/cost, it basically follows the fast strategy, except unlike the fast strategy, it doesn't try to prevent the window from jumping.

So in addition to doing the window adjustment in the fast strategy, we can also do it for the safe strategy when it fails and returns nil.

[justinbarclay]

Hey, this looks great. Thanks for the PR 😄