patch 8.2.1677: memory access errors when calling setloclist() in autocommand

Problem:    Memory access errors when calling setloclist() in an autocommand.
Solution:   Give an error if the list was changed unexpectedly. (closes #6946)

src/testdir/test_quickfix.vim

@@ -1430,6 +1430,30 @@ func Test_quickfix_was_changed_by_autocmd()
   call XquickfixChangedByAutocmd('l')
 endfunc
 
+func Test_setloclist_in_autocommand()
+  call writefile(['test1', 'test2'], 'Xfile')
+  edit Xfile
+  let s:bufnr = bufnr()
+  call setloclist(1,
+        \ [{'bufnr' : s:bufnr, 'lnum' : 1, 'text' : 'test1'},
+        \  {'bufnr' : s:bufnr, 'lnum' : 2, 'text' : 'test2'}])
+
+  augroup Test_LocList
+    au!
+    autocmd BufEnter * call setloclist(1,
+          \ [{'bufnr' : s:bufnr, 'lnum' : 1, 'text' : 'test1'},
+          \  {'bufnr' : s:bufnr, 'lnum' : 2, 'text' : 'test2'}], 'r')
+  augroup END
+
+  lopen
+  call assert_fails('exe "normal j\<CR>"', 'E926:')
+
+  augroup Test_LocList
+    au!
+  augroup END
+  call delete('Xfile')
+endfunc
+
 func Test_caddbuffer_to_empty()
   helpgr quickfix
   call setqflist([], 'r')