patch 8.1.0488: using freed memory in quickfix code

Problem:    Using freed memory in quickfix code. (Dominique Pelle)
Solution:   Add the quickfix_busy() flag to postpone deleting quickfix lists
            until it is safe. (Yegappan Lakshmanan, closes #3538)
diff --git a/src/version.c b/src/version.c
index 4189eb0..7618740 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    488,
+/**/
     487,
 /**/
     486,