refactor: use CLEAR_FIELD and CLEAR_POINTER macros (#19709)

vim-patch:8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
a80faa8930

src/nvim/screen.c

@@ -2399,7 +2399,7 @@ static int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, bool noc
     }
   }
 
-  memset(sattrs, 0, sizeof(sattrs));
+  CLEAR_FIELD(sattrs);
   num_signs = buf_get_signattrs(wp->w_buffer, lnum, sattrs);
   decor_redraw_signs(buf, lnum - 1, &num_signs, sattrs);