patch 9.1.0021: i_CTRL-R- doesn't work for multibyte chars in Replace mode

patch 9.1.0021: i_CTRL-R- doesn't work for multibyte chars in Replace mode

Problem:  i_CTRL-R- doesn't work for multibyte chars in Replace mode,
          Coverity complains missing return value for u_save_cursor()
Solution: Use mb_charlen() and del_chars() instead, handle failure mode
          for u_save_cursor() correctly (@zeertzjq)

closes: #13846

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>