fixes #31002 #22919 and possibly a class of crashes triggered when msg_scroll_flush() is called during a screen resize.

The root cause of the crashes is that msg_scroll_flush() assumes that the message layout state is consistent with the current geometry, however, during screen_resize() this assumption is invalid:

The resize operation updates geometry in several steps, screen_resize() triggers VimResized autocmds before the resize is finished, those autocmds may execute :redraw, screenstring(), and any other function that calls into the UI layer.

These paths eventually call msg_scroll_flush(), but the message grid is still using stale positions and offsets, Which may triggers the following asserts:

row >= 0 (reproducible)
pos_delta >= 0 ( not always reproducible see chat on issue #32319)
to_scroll >= 0 ( not always reproducible see chat on issue #22919)

while trying to fix these issues I was placing several if statement, in different locations in the codebase, when i realized
that it could be safer, and a lot cleaner to make msg_scroll_flush() a no-op while the screen is resizing:

if (resizing_screen) {
  msg_scrolled_at_flush = msg_scrolled;
  msg_grid_scroll_discount = 0;
  return;
}

This ensures:

• no geometry-dependent scroll math runs during resize
• no interaction with stale msg_grid_pos

and once the resize completes, msg_grid_set_pos(),compute_cmdrow(),update_screen() ,msg_grid_validate() recompute the correct message layout, and normal scrolling resumes safely.