terminal: Only wipe buffer terminal_close wasn't called

After @250aca4f8938 it is possible that terminal_close will be called without
invoking the close_cb(which normally destroys the terminal structure). If this
happens, the terminal buffer will already be deleted so there's no need to call
`bwipeout!`.

src/nvim/terminal.c

@@ -423,7 +423,9 @@ end:
   ui_busy_stop();
   if (close) {
     term->opts.close_cb(term->opts.data);
-    do_cmdline_cmd("bwipeout!");
+    if (term->buf) {
+      do_cmdline_cmd("bwipeout!");
+    }
   }
 }