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/evalwindow.c b/src/evalwindow.c
index fde90f2..7d035d5 100644
--- a/src/evalwindow.c
+++ b/src/evalwindow.c
@@ -1076,7 +1076,7 @@
     else if (WIN_IS_POPUP(wp))
 	rettv->vval.v_string = vim_strsave((char_u *)"popup");
 #endif
-    else if (wp == curwin && cmdwin_type != 0)
+    else if (wp == cmdwin_win)
 	rettv->vval.v_string = vim_strsave((char_u *)"command");
 #ifdef FEAT_QUICKFIX
     else if (bt_quickfix(wp->w_buffer))