patch 8.0.0565: using freed memory in :caddbuf

Problem:    Using freed memory in :caddbuf after clearing quickfix list.
            (Dominique Pelle)
Solution:   Set qf_last to NULL.
diff --git a/src/quickfix.c b/src/quickfix.c
index 58fcd11..d2921b2 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -2750,6 +2750,7 @@
     vim_free(qi->qf_lists[idx].qf_title);
     qi->qf_lists[idx].qf_title = NULL;
     qi->qf_lists[idx].qf_index = 0;
+    qi->qf_lists[idx].qf_last = NULL;
 
     qf_clean_dir_stack(&qi->qf_dir_stack);
     qi->qf_directory = NULL;