patch 8.1.0469: too often indexing in qf_lists[]

Problem:    Too often indexing in qf_lists[].
Solution:   Use a qf_list_T pointer. (Yegappan Lakshmanan)

src/testdir/test_quickfix.vim

@@ -558,6 +558,8 @@ func s:test_xhelpgrep(cchar)
 
   " Search for non existing help string
   call assert_fails('Xhelpgrep a1b2c3', 'E480:')
+  " Invalid regular expression
+  call assert_fails('Xhelpgrep \@<!', 'E480:')
 endfunc
 
 func Test_helpgrep()