fix(smoothscroll): crash when resizing to textoff with showbreak

vim-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: vim/vim#20012

AI-assisted: Codex

https://github.com/vim/vim/commit/0e31fb024c846e36bb0d26d01ff179a0d1b3eae4

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