screen.c: add update_window_hl to special redrawing entrypoints

The following (run as a script) used to cause a crash due to :sign using a
special redraw (not updating nvim's specific highlight data structures)
without proper redraw first, as split just flags for redraw later.

set cursorline
sign define piet text=>> texthl=Search
split
sign place 3 line=2 name=piet buffer=1

src/nvim/screen.c

@@ -507,6 +507,7 @@ void update_single_line(win_T *wp, linenr_T lnum)
         init_search_hl(wp);
         start_search_hl();
         prepare_search_hl(wp, lnum);
+        update_window_hl(wp, false);
         win_line(wp, lnum, row, row + wp->w_lines[j].wl_size, false);
         end_search_hl();
         break;
@@ -579,6 +580,7 @@ void update_debug_sign(buf_T *buf, linenr_T lnum)
 
     FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
       if (wp->w_redr_type != 0) {
+        update_window_hl(wp, wp->w_redr_type >= NOT_VALID);
         win_update(wp);
       }
       if (wp->w_redr_status) {