Under insert mode, when inserting a normal character in front of
a wide character, the affected region is shifted to the right by
one cell. However, the empty cell is reset as if being a part of a
wide character, causing the following cell being mishandled as a
dummy cell.
To reproduce the bug:
printf '\033[4h' # set MODE_INSERT
printf 妳好
printf '\033[4D'
printf 'x'
printf '\033[4l\n'
