patch 8.1.1521: when a popup window is closed the buffer remains

Problem:    When a popup window is closed the buffer remains.
Solution:   Wipe out the buffer.
diff --git a/src/version.c b/src/version.c
index 8f3d086..28877cb 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1521,
+/**/
     1520,
 /**/
     1519,