patch 9.1.0047: issues with temp curwin/buf while cmdwin is open

Problem:  Things that temporarily change/restore curwin/buf (e.g:
          win_execute, some autocmds) may break assumptions that
          curwin/buf is the cmdwin when "cmdwin_type != 0", causing
          issues.

Solution: Expose the cmdwin's real win/buf and check that instead. Also
          try to ensure these variables are NULL if "cmdwin_type == 0",
          allowing them to be used directly in most cases without
          checking cmdwin_type. (Sean Dewar)

Alternatively, we could ban win_execute in the cmdwin and audit all places that
temporarily change/restore curwin/buf, but I didn't notice any problems arising
from allowing this (standard cmdwin restrictions still apply, so things that may
actually break the cmdwin are still forbidden).

closes: #12819

Signed-off-by: Sean Dewar <seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/window.c

@@ -3375,6 +3375,8 @@ win_free_all(void)
 
     // avoid an error for switching tabpage with the cmdline window open
     cmdwin_type = 0;
+    cmdwin_buf = NULL;
+    cmdwin_win = NULL;
 
     while (first_tabpage->tp_next != NULL)
 	tabpage_close(TRUE);