patch 9.1.0048: Abort opening cmdwin if autocmds screw things up

Problem:  Autocmds triggered from opening the cmdwin (in win_split and
          do_ecmd) can cause issues such as E199, as the current checks
          are insufficient.

Solution: Commands executed from the cmdwin apply to the old curwin/buf,
          so they should be kept in a "suspended" state; abort if
          they've changed. Also abort if cmdwin/buf was tampered with,
          and check that curwin is correct. Try to clean up the cmdwin
          buffer (only if hidden and non-current to simplify things; the
          same approach is used when closing cmdwin normally), and add a
          beep. (Sean Dewar)

It'd be nice to also check that curwin was *really* created by win_split, as
autocommands can change curwin before it returns (so it can't be assumed to be
that of the split); for now, this means that the cmdwin may not be the botwin in
that case, which is probably OK.

closes: #12819

Signed-off-by: Sean Dewar <seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/window.c b/src/window.c
index 2e12043..fda42e9 100644
--- a/src/window.c
+++ b/src/window.c
@@ -2485,7 +2485,7 @@
  * "aucmd_win[]").
  * Returns FALSE if there is a window, possibly in another tab page.
  */
-    static int
+    int
 last_window(void)
 {
     return (one_window() && first_tabpage->tp_next == NULL);