fix clippy: call next_back instead of last on DoubleEndedIterator

[WindSoilder]

Fixes clippy(version 0.1.86) error:

error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
   --> src/core_editor/line_buffer.rs:474:14
    |
474 |             .last()
    |              ^^^^^^ help: try: `next_back()`

[sholderbach]

Thanks