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/textformat.c b/src/textformat.c
index a1a3e16..5a52950 100644
--- a/src/textformat.c
+++ b/src/textformat.c
@@ -795,7 +795,7 @@
 	// The width is the window width minus 'wrapmargin' minus all the
 	// things that add to the margin.
 	textwidth = curwin->w_width - curbuf->b_p_wm;
-	if (cmdwin_type != 0)
+	if (curbuf == cmdwin_buf)
 	    textwidth -= 1;
 #ifdef FEAT_FOLDING
 	textwidth -= curwin->w_p_fdc;