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/highlight.c b/src/highlight.c
index 1a4c76d..8c1ad80 100644
--- a/src/highlight.c
+++ b/src/highlight.c
@@ -262,6 +262,9 @@
     "default link PmenuMatchSel PmenuSel",
     "default link PmenuExtra Pmenu",
     "default link PmenuExtraSel PmenuSel",
+    "default link PopupSelected PmenuSel",
+    "default link MessageWindow WarningMsg",
+    "default link PopupNotification WarningMsg",
     CENT("Normal cterm=NONE", "Normal gui=NONE"),
     NULL
 };