patch 9.0.0965: using one window for executing autocommands is insufficient

Problem:    Using one window for executing autocommands is insufficient.
Solution:   Use up to five windows for executing autocommands.
diff --git a/src/fileio.c b/src/fileio.c
index 9916de8..1ee08e2 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4369,8 +4369,14 @@
     int		flags = READ_NEW;
     int		prepped = OK;
 
-    // set curwin/curbuf for "buf" and save some things
+    // Set curwin/curbuf for "buf" and save some things.
     aucmd_prepbuf(&aco, buf);
+    if (curbuf != buf)
+    {
+	// Failed to find a window for "buf", it is dangerous to continue,
+	// better bail out.
+	return;
+    }
 
     // Unless reload_options is set, we only want to read the text from the
     // file, not reset the syntax highlighting, clear marks, diff status, etc.