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>

src/insexpand.c

@@ -3075,7 +3075,8 @@ info_add_completion_info(list_T *li)
 
     // Skip the element with the CP_ORIGINAL_TEXT flag at the beginning, in case of
     // forward completion, or at the end, in case of backward completion.
-    match = forward ? match->cp_next : (compl_no_select && match_at_original_text(match) ? match->cp_prev : match->cp_prev->cp_prev);
+    match = forward || match->cp_prev == NULL ? match->cp_next :
+	(compl_no_select && match_at_original_text(match) ? match->cp_prev : match->cp_prev->cp_prev);
 
     while (match != NULL && !match_at_original_text(match))
     {

src/testdir/test_ins_complete.vim

@@ -2359,4 +2359,18 @@ func Test_complete_info_index()
   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

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    65,
 /**/
     64,
 /**/