patch 8.1.0865: when 'listchars' only contains "nbsp:X" it does not work

Problem:    When 'listchars' only contains "nbsp:X" it does not work.
Solution:   Set extra_check when lcs_nbsp is set. (Ralf Schandl, closes #3889)
diff --git a/src/screen.c b/src/screen.c
index 1370773..caf6e2c 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -1164,8 +1164,6 @@
 	    mod_bot = wp->w_redraw_bot + 1;
 	else
 	    mod_bot = 0;
-	wp->w_redraw_top = 0;	/* reset for next time */
-	wp->w_redraw_bot = 0;
 	if (buf->b_mod_set)
 	{
 	    if (mod_top == 0 || mod_top > buf->b_mod_top)
@@ -1277,6 +1275,8 @@
 	if (mod_top != 0 && buf->b_mod_xlines != 0 && wp->w_p_nu)
 	    mod_bot = MAXLNUM;
     }
+    wp->w_redraw_top = 0;	// reset for next time
+    wp->w_redraw_bot = 0;
 
     /*
      * When only displaying the lines at the top, set top_end.  Used when
@@ -3498,7 +3498,7 @@
 
     if (wp->w_p_list)
     {
-	if (lcs_space || lcs_trail)
+	if (lcs_space || lcs_trail || lcs_nbsp)
 	    extra_check = TRUE;
 	/* find start of trailing whitespace */
 	if (lcs_trail)