patch 8.1.1612: cannot show an existing buffer in a popup window

Problem:    Cannot show an existing buffer in a popup window.
Solution:   Support buffer number argument in popup_create().
diff --git a/src/normal.c b/src/normal.c
index 2015fd8..5c9f929 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -4525,7 +4525,7 @@
 	if (wp == NULL)
 	    return;
 #ifdef FEAT_TEXT_PROP
-	if (bt_popup(wp->w_buffer) && !wp->w_has_scrollbar)
+	if (WIN_IS_POPUP(wp) && !wp->w_has_scrollbar)
 	    return;
 #endif
 	curwin = wp;
@@ -4560,7 +4560,7 @@
 	    nv_scroll_line(cap);
 	}
 #ifdef FEAT_TEXT_PROP
-	if (bt_popup(curwin->w_buffer))
+	if (WIN_IS_POPUP(curwin))
 	    popup_set_firstline(curwin);
 #endif
     }