Panic when pressing Shift + Tab in an empty line

[Fulgen301]

When Shift+Tab is pressed at the beginning of an empty line, edit crashes with a panic.

edit version: e8d40f6
Built with: rustc 1.89.0-nightly (60dabef95 2025-05-19)

thread 'main' panicked at src\buffer\mod.rs:1995:27:
index out of bounds: the len is 0 but the index is 0
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/60dabef95a3de3ec974dcb50926e4bfe743f078f/library\std\src\panicking.rs:697
   1: core::panicking::panic_fmt
             at /rustc/60dabef95a3de3ec974dcb50926e4bfe743f078f/library\core\src\panicking.rs:75
   2: core::panicking::panic_bounds_check
             at /rustc/60dabef95a3de3ec974dcb50926e4bfe743f078f/library\core\src\panicking.rs:280
   3: core::slice::index::impl$2::index<u8>
             at C:\Users\<username>\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\slice\index.rs:274
   4: alloc::vec::impl$13::index<u8,usize,alloc::alloc::Global>
             at C:\Users\<username>\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\alloc\src\vec\mod.rs:3376
   5: edit::buffer::TextBuffer::unindent
             at .\src\buffer\mod.rs:1995
   6: edit::tui::Context::textarea_handle_input
             at .\src\tui.rs:2268
   7: edit::tui::Context::textarea_internal
             at .\src\tui.rs:2068
   8: edit::tui::Context::textarea
             at .\src\tui.rs:1981
   9: edit::draw_editor::draw_editor
             at .\src\bin\edit\draw_editor.rs:27
  10: edit::draw
             at .\src\bin\edit\main.rs:289
  11: edit::run
             at .\src\bin\edit\main.rs:137
  12: edit::main
             at .\src\bin\edit\main.rs:50
  13: core::ops::function::FnOnce::call_once<std::process::ExitCode (*)(),tuple$<> >
             at C:\Users\<username>\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:250

[marcos-venicius]

Heey! I just created a PR #137 to solve this issue