patch 9.1.1083: setreg() doesn't correctly handle mbyte chars in blockwise mode

patch 9.1.1083: setreg() doesn't correctly handle mbyte chars in blockwise mode

Problem:  setreg() doesn't correctly handle mbyte chars in blockwise
          mode
Solution: use mb_ptr2len_len function pointer (Yee Cheng Chin)

setreg() will automatically calculate the width when a blockwise mode is
specified, but it does not properly calculate the line widths of mbyte
characters when value is passed as newline-terminated string. It does
work when value is passed as a list of lines though.

Fix this by properly using the mbyte function pointer to increment the
loop counter.

closes: #16596

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>