patch 9.1.0132: "C" doesn't include composing chars with 'virtualedit'

patch 9.1.0132: "C" doesn't include composing chars with 'virtualedit'

Problem:  using "C" and 've=all' set, doesn't include composing chars
          when changing a line, keeps the composing chars for whatever
          is typed afterwards.
Solution: Use mb_head_off() and mb_ptr2len() instead of mb_tail_off().
          (zeertzjq)

closes: #14083

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