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/popupwin.c b/src/popupwin.c
index 2bac5c8..0b360df 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -1550,8 +1550,7 @@
     {
 	// create a new buffer associated with the popup
 	new_buffer = TRUE;
-	buf = buflist_new(NULL, NULL, (linenr_T)0,
-						 BLN_NEW|BLN_LISTED|BLN_DUMMY);
+	buf = buflist_new(NULL, NULL, (linenr_T)0, BLN_NEW|BLN_DUMMY|BLN_REUSE);
 	if (buf == NULL)
 	    return NULL;
 	ml_open(buf);