vim: Fix panic due to overflow when scrolling (#16029)

When setting `"vertical_scroll_margin": 99` or other high values this
can lead to a panic that crashes Zed.

Release Notes:

- vim: Fixed a possible panic that could happen when using a very high
value for `vertical_scroll_margin` that exceeded the number of visible
lines on the screen.

Co-authored-by: Bennet <bennet@zed.dev>