patch 8.1.1615: crash when passing buffer number to popup_create()

Problem:    Crash when passing buffer number to popup_create(). (Yasuhiro
            Matsumoto)
Solution:   Initialze the window properly.
diff --git a/src/popupwin.c b/src/popupwin.c
index 3af35b6..d988cba 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -1056,8 +1056,7 @@
     {
 	// use existing buffer
 	new_buffer = FALSE;
-	wp->w_buffer = buf;
-	++buf->b_nwindows;
+	win_init_popup_win(wp, buf);
 	buffer_ensure_loaded(buf);
     }
     else