[Low Priority] Panic if opening large amounts of files with --vsplit and --hsplit

[NikitaRevenco]

When we try open 200 files in a split:

hx --vsplit {1..200}

Helix will panic with the following message:

thread 'main' panicked at helix-tui/src/buffer.rs:516:21:
index out of bounds: the len is 4788 but the index is 4788
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

There should be some kind of error message when trying to pass a large number to --vsplit and --hsplit

[kirawi]

#4642