patch 8.1.1445: popup window border highlight not implemented yet

Problem:    Popup window border highlight not implemented yet.
Solution:   Implement the "borderhighlight" option.
diff --git a/src/window.c b/src/window.c
index b4ab11b..e15b311 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4846,6 +4846,8 @@
 #endif
 #ifdef FEAT_TEXT_PROP
     free_callback(&wp->w_filter_cb);
+    for (i = 0; i < 4; ++i)
+	VIM_CLEAR(wp->w_border_highlight[i]);
 #endif
 
 #ifdef FEAT_SYN_HL