patch 8.0.1420: accessing freed memory in vimgrep

Problem:    Accessing freed memory in vimgrep.
Solution:   Check that the quickfix list is still valid. (Yegappan Lakshmanan,
            closes #2474)
diff --git a/src/version.c b/src/version.c
index 6ae2135..1a217e7 100644
--- a/src/version.c
+++ b/src/version.c
@@ -772,6 +772,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1420,
+/**/
     1419,
 /**/
     1418,