patch 8.0.1412: using free memory using setloclist()

Problem:    Using free memory using setloclist(). (Dominique Pelle)
Solution:   Mark location list context as still in use when needed. (Yegappan
            Lakshmanan, closes #2462)
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index 2204574..8d0c198 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -3017,3 +3017,17 @@
   call Xqftick_tests('c')
   call Xqftick_tests('l')
 endfunc
+
+" The following test used to crash Vim.
+" Open the location list window and close the regular window associated with
+" the location list. When the garbage collection runs now, it incorrectly
+" marks the location list context as not in use and frees the context.
+func Test_ll_window_ctx()
+  call setloclist(0, [], 'f')
+  call setloclist(0, [], 'a', {'context' : []})
+  lopen | only
+  call test_garbagecollect_now()
+  echo getloclist(0, {'context' : 1}).context
+  enew | only
+endfunc
+