patch 9.1.0065: Segfault with CompleteChanged autocommand

Problem:  Segfault with CompleteChanged autocommand
          (markonm )
Solution: Test match->cp_prev for being NULL before accessing it

closes: #13929

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_ins_complete.vim b/src/testdir/test_ins_complete.vim
index 61ddcd3..1b6eafe 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -2359,4 +2359,18 @@
   bwipe!
 endfunc
 
-" vim: shiftwidth=2 sts=2 expandtab
+func Test_complete_changed_complete_info()
+  CheckRunVimInTerminal
+  " this used to crash vim, see #13929
+  let lines =<< trim END
+    set completeopt=menuone
+    autocmd CompleteChanged * call complete_info(['items'])
+    call feedkeys("iii\<cr>\<c-p>")
+  END
+  call writefile(lines, 'Xsegfault', 'D')
+  let buf = RunVimInTerminal('-S Xsegfault', #{rows: 5})
+  call WaitForAssert({-> assert_match('^ii', term_getline(buf, 1))}, 1000)
+  call StopVimInTerminal(buf)
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab nofoldenable