patch 8.1.0489: crash when autocmd clears vimpgrep location list

Problem:    Crash when autocmd clears vimpgrep location list.
Solution:   Return from qf_jump_edit_buffer() early. (Yegappan Lakshmanan)
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index 390a231..01a950f 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -3255,6 +3255,17 @@
   augroup QF_Test
     au!
   augroup END
+
+  new | only
+  augroup QF_Test
+    au!
+    au BufEnter * call setloclist(0, [], 'r')
+  augroup END
+  call assert_fails('lvimgrep Test_lvimgrep_crash *', 'E926:')
+  augroup QF_Test
+    au!
+  augroup END
+
   enew | only
 endfunc
 
@@ -3337,6 +3348,37 @@
   call assert_equal('help', &filetype)
   call assert_equal(1, getloclist(0, {'nr' : '$'}).nr)
   au! QuickFixCmdPost
+
+  new | only
+  augroup QF_Test
+    au!
+    au BufEnter * call setqflist([], 'f')
+  augroup END
+  call assert_fails('helpgrep quickfix', 'E925:')
+  augroup QF_Test
+    au! BufEnter
+  augroup END
+
+  new | only
+  augroup QF_Test
+    au!
+    au BufEnter * call setqflist([], 'r')
+  augroup END
+  call assert_fails('helpgrep quickfix', 'E925:')
+  augroup QF_Test
+    au! BufEnter
+  augroup END
+
+  new | only
+  augroup QF_Test
+    au!
+    au BufEnter * call setloclist(0, [], 'r')
+  augroup END
+  call assert_fails('lhelpgrep quickfix', 'E926:')
+  augroup QF_Test
+    au! BufEnter
+  augroup END
+
   new | only
 endfunc