patch 9.1.1123: popup hi groups not falling back to defaults

Problem:  Highlight groups PopupSelected/PopupNotification/
          MessageWindow are supposed to fall back to default highlight
          groups if they are not defined. However, once a colorscheme
          has defined them, switching to another colorscheme that
          doesn't do so will leave behind a cleared colorscheme, which
          causes the fallback to fail.

Solution: Set up default links to the relevant fallback highlight
          groups, which makes sure a `:hi clear` command will reset the
          state properly (Yee Cheng Chin).

closes: #16676

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/popupwin.c b/src/popupwin.c
index 4045490..c84c079 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -696,9 +696,6 @@
 	if (!sign_exists_by_name(sign_name))
 	{
 	    char *linehl = "PopupSelected";
-
-	    if (syn_name2id((char_u *)linehl) == 0)
-		linehl = "PmenuSel";
 	    sign_define_by_name(sign_name, NULL, (char_u *)linehl, NULL, NULL, NULL,
 		    NULL, SIGN_DEF_PRIO);
 	}
@@ -2006,10 +2003,8 @@
 {
     char    *hiname = type == TYPE_MESSAGE_WIN
 				       ? "MessageWindow" : "PopupNotification";
-    int		nr = syn_name2id((char_u *)hiname);
-
     set_string_option_direct_in_win(wp, (char_u *)"wincolor", -1,
-		(char_u *)(nr == 0 ? "WarningMsg" : hiname),
+		(char_u *)hiname,
 		OPT_FREE|OPT_LOCAL, 0);
 }