multi_buffer: Fix a panic when expanding an excerpt with the cursor at the end (#17955)

This PR fixes a panic when expanding an excerpt within a multibuffer
that could occur when the cursor was at the end of the buffer.

You can reproduce this by opening a multibuffer, putting your cursor at
the very end of that buffer, and then expanding the excerpt (Shift +
Enter).

Release Notes:

- Fixed a panic that could occur when expanding an excerpt within a
multibuffer when the cursor was at the end of the excerpt.

Co-authored-by: Antonio <antonio@zed.dev>