patch 7.4.1647

Problem:    Using freed memory after setqflist() and ":caddbuffer".  (Dominique)
Solution:   Set qf_ptr when adding the first item to the quickfix list.

src/quickfix.c

@@ -1027,6 +1027,8 @@ qf_add_entry(
 				/* first element in the list */
     {
 	qi->qf_lists[qi->qf_curlist].qf_start = qfp;
+	qi->qf_lists[qi->qf_curlist].qf_ptr = qfp;
+	qi->qf_lists[qi->qf_curlist].qf_index = 0;
 	qfp->qf_prev = qfp;	/* first element points to itself */
     }
     else
@@ -4113,6 +4115,7 @@ set_errorlist(
     else
 	qi->qf_lists[qi->qf_curlist].qf_nonevalid = FALSE;
     qi->qf_lists[qi->qf_curlist].qf_ptr = qi->qf_lists[qi->qf_curlist].qf_start;
+    if (qi->qf_lists[qi->qf_curlist].qf_count > 0)
 	qi->qf_lists[qi->qf_curlist].qf_index = 1;
 
 #ifdef FEAT_WINDOWS

src/testdir/test_quickfix.vim

@@ -679,3 +679,11 @@ function Test_quickfix_was_changed_by_autocmd()
   call XquickfixChangedByAutocmd('c')
   call XquickfixChangedByAutocmd('l')
 endfunction
+
+func Test_caddbuffer_to_empty()
+  helpgr quickfix
+  call setqflist([], 'r')
+  cad
+  call assert_fails('cn', 'E553:')
+  quit!
+endfunc

src/version.c

@@ -748,6 +748,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1647,
 /**/
     1646,
 /**/