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/ex_getln.c b/src/ex_getln.c
index f2a960e..e20842b 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -4494,8 +4494,9 @@
     // Don't let quitting the More prompt make this fail.
     got_int = FALSE;
 
-    // Set "cmdwin_type" before any autocommands may mess things up.
+    // Set "cmdwin_..." variables before any autocommands may mess things up.
     cmdwin_type = get_cmdline_type();
+    cmdwin_win = curwin;
 
     // Create the command-line buffer empty.
     if (do_ecmd(0, NULL, NULL, NULL, ECMD_ONE, ECMD_HIDE, NULL) == FAIL)
@@ -4504,8 +4505,10 @@
 	win_close(curwin, TRUE);
 	ga_clear(&winsizes);
 	cmdwin_type = 0;
+	cmdwin_win = NULL;
 	return Ctrl_C;
     }
+    cmdwin_buf = curbuf;
 
     apply_autocmds(EVENT_BUFFILEPRE, NULL, NULL, FALSE, curbuf);
     (void)setfname(curbuf, (char_u *)_("[Command Line]"), NULL, TRUE);
@@ -4615,6 +4618,8 @@
 # endif
 
     cmdwin_type = 0;
+    cmdwin_buf = NULL;
+    cmdwin_win = NULL;
     exmode_active = save_exmode;
 
     // Safety check: The old window or buffer was deleted: It's a bug when