Fixed a crash of editor when deleting multiple bytes characters in prompt

[mirao]

It fixes this issue:
If you enter a multiple bytes character, e.g. ž or 😒 in prompt after Ctrl + F or Ctrl + S and then delete the character by Backspace, editor crashes:

thread 'main' panicked at 'assertion failed: self.is_char_boundary(new_len)', /rustc/09c42c45858d5f3aedfa670698275303a3d19afa/library/alloc/src/string.rs:1193:13

[pflenker]

Thanks! I'll leave this open and link to it from the tutorial.