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)

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1420,
 /**/
     1419,
 /**/