patch 9.0.0490: using freed memory with cmdwin and BufEnter autocmd

Problem:    Using freed memory with cmdwin and BufEnter autocmd.
Solution:   Make sure pointer to b_p_iminsert is still valid.
diff --git a/src/testdir/test_cmdwin.vim b/src/testdir/test_cmdwin.vim
index d62673a..fe849bc 100644
--- a/src/testdir/test_cmdwin.vim
+++ b/src/testdir/test_cmdwin.vim
@@ -378,5 +378,15 @@
   call assert_equal('" bar', @:)
 endfunc
 
+" This was using a pointer to a freed buffer
+func Test_cmdwin_freed_buffer_ptr()
+  au BufEnter * next 0| file 
+  edit 0
+  silent! norm q/
+
+  au! BufEnter
+  bwipe!
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab