fix `arithmetic operation overflowed`

[quininer]

steps to reproduce:

1. iota --vi README.md
2. jjjbk

"move one word backward" behavior seems wrong.

     Running `target/debug/iota --vi README.md`
thread '<main>' panicked at 'arithmetic operation overflowed', src/iota/buffer.rs:310
stack backtrace:
   1:     0x56052a9ab210 - sys::backtrace::tracing::imp::write::h1c67167e36198df5Qwu
   2:     0x56052a9ae5ab - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.43471
   3:     0x56052a9ae218 - panicking::default_handler::h9023da11475bdb1dA3y
   4:     0x56052a99fbac - sys_common::unwind::begin_unwind_inner::hf5ab264a07c201cdFlt
   5:     0x56052a99fff8 - sys_common::unwind::begin_unwind_fmt::hd9d6d1b19c7960d2Lkt
   6:     0x56052a9aa701 - rust_begin_unwind
   7:     0x56052a9d710f - panicking::panic_fmt::h62352977d16cc5446YL
   8:     0x56052a9d73e8 - panicking::panic::he36bffa3158bb900DXL
   9:     0x56052a85939c - buffer::Buffer::get_line_index_backward::hccf2332e51aa6c06Qwa
                        at src/iota/buffer.rs:310
  10:     0x56052a857955 - buffer::Buffer::get_line_index::h9dd919bd9b814105Nta
                        at src/iota/buffer.rs:225
  11:     0x56052a85722e - buffer::Buffer::get_object_index::h7ad693721815990eXqa
                        at src/iota/buffer.rs:141
  12:     0x56052a85dcce - buffer::Buffer::set_mark_to_object::h13040953f40f6bc71Na
                        at src/iota/buffer.rs:508
  13:     0x56052a86ccd4 - view::View::move_mark::h2283659049b22eccWJb
                        at src/iota/view.rs:208
  14:     0x56052a84d80a - editor::Editor<'e, T>::handle_instruction::h16327850811678013980
                        at src/iota/editor.rs:174
  15:     0x56052a84d245 - editor::Editor<'e, T>::handle_command::h15209287566117992244
                        at src/iota/editor.rs:161
  16:     0x56052a84c566 - editor::Editor<'e, T>::handle_key_event::h1659430630403727501
                        at src/iota/editor.rs:86
  17:     0x56052a848ef6 - editor::Editor<'e, T>::start::h5983973856314194529
                        at src/iota/editor.rs:225
  18:     0x56052a838fc0 - main::h664c38f33474903aeda
                        at src/main.rs:77
  19:     0x56052a9ade74 - sys_common::unwind::try::try_fn::h14839783617401448156
  20:     0x56052a9aa68b - __rust_try
  21:     0x56052a9ad8fe - rt::lang_start::h5fa2ad55c2a21093GVy
  22:     0x56052a84e079 - main
  23:     0x7f9529ce260f - __libc_start_main
  24:     0x56052a8384b8 - _start
  25:                0x0 - <unknown>
Process didn't exit successfully: `target/debug/iota --vi README.md` (exit code: 101)

[gchp]

Hey @quininer! Thank you for taking the time to submit this.

While this certainly does fix the crash you mention, I feel it doesn't fix it in the right way. What this does is reset the cursor back to the start of buffer - regardless of your previous position.

I'm wondering if a better approach to this issue would be to fix get_word_index_backward and get_word_index_forward in buffer.rs instead? These are the two methods for handling word-based movement, and causing the incorrect position after pressing b. If those two methods were performing correctly, we may not need this patch.

On the other hand, once those methods are performing correctly, it might be the case that we definitely need this patch, I'm just not sure yet. So I'm going to hold off on this for the moment, until the word-based movement is working properly. I'll open a ticket for that...

Thanks again!

[gchp]

@quininer this has been resolved by another commit, so I'm going to close this. But I really appreciate you taking the time to put this patch together. Thank you!