patch 9.0.0590: after exiting Insert mode spelling not checked in next line

Problem:    After exiting Insert mode spelling is not checked in the next
            line.
Solution:   When spelling is enabled redraw the next line after exiting Insert
            mode in case the spell highlight needs updating.
diff --git a/src/spell.c b/src/spell.c
index 08f627d..628814f 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -1238,18 +1238,27 @@
 }
 
 /*
- * Return TRUE if spell checking is not enabled.
+ * Return TRUE if spell checking is enabled for "wp".
+ */
+    int
+spell_check_window(win_T *wp)
+{
+    return wp->w_p_spell
+		&& *wp->w_s->b_p_spl != NUL
+		&& wp->w_s->b_langp.ga_len > 0
+		&& *(char **)(wp->w_s->b_langp.ga_data) != NULL;
+}
+
+/*
+ * Return TRUE and give an error if spell checking is not enabled.
  */
     static int
 no_spell_checking(win_T *wp)
 {
-    if (!wp->w_p_spell || *wp->w_s->b_p_spl == NUL
-					 || wp->w_s->b_langp.ga_len == 0)
-    {
-	emsg(_(e_spell_checking_is_not_possible));
-	return TRUE;
-    }
-    return FALSE;
+    if (spell_check_window(wp))
+	return FALSE;
+    emsg(_(e_spell_checking_is_not_possible));
+    return TRUE;
 }
 
 /*