patch 8.1.1908: every popup window consumes a buffer number

Problem:    Every popup window consumes a buffer number.
Solution:   Recycle buffers only used for popup windows.  Do not list popup
            window buffers.
diff --git a/src/window.c b/src/window.c
index fabd196..8da5ed0 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4970,7 +4970,7 @@
 win_free_popup(win_T *win)
 {
     if (bt_popup(win->w_buffer))
-	win_close_buffer(win, DOBUF_WIPE, FALSE);
+	win_close_buffer(win, DOBUF_WIPE_REUSE, FALSE);
     else
 	close_buffer(win, win->w_buffer, 0, FALSE);
 # if defined(FEAT_TIMERS)