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>
diff --git a/src/globals.h b/src/globals.h
index b74ef35..dd3d208 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1686,6 +1686,8 @@
 
 EXTERN int	cmdwin_type INIT(= 0);	// type of cmdline window or 0
 EXTERN int	cmdwin_result INIT(= 0); // result of cmdline window or 0
+EXTERN buf_T	*cmdwin_buf INIT(= NULL); // buffer of cmdline window or NULL
+EXTERN win_T	*cmdwin_win INIT(= NULL); // window of cmdline window or NULL
 
 EXTERN char_u no_lines_msg[]	INIT(= N_("--No lines in buffer--"));