patch 9.2.0362: division by zero with smoothscroll and small windows

patch 9.2.0362: division by zero with smoothscroll and small windows

Problem:  Resizing a smoothscrolled wrapped window to its textoff width
          with 'showbreak' can leave wrapped continuation lines with
          zero text width. win_lbr_chartabsize() still runs the partial max_head_vcol calculation in
          that state and divides by width2, crashing during redraw.
Solution: Skip that partial head calculation when the wrapped
          continuation width is zero, matching the other width2 guards
          in charset.c (Jaehwang Jung)

closes: #20012

AI-assisted: Codex

Signed-off-by: Jaehwang Jung <tomtomjhj@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>