Fix window destruction

Using F6 to close the debugger tab caused the message `E855: Autocommands caused command to abort` and sometimes caused a `vim.error`:
```
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "F:\Settings\home\.vim\vundle\vdebug\plugin/python/start_vdebug.py", line 171, in close
    self.runner.close()
  File "F:\Settings\home\.vim\vundle\vdebug\plugin/python\vdebug\runner.py", line 338, in close
    self.ui.close()
  File "F:\Settings\home\.vim\vundle\vdebug\plugin/python\vdebug\ui\vimui.py", line 165, in close
    self.statuswin.destroy()
  File "F:\Settings\home\.vim\vundle\vdebug\plugin/python\vdebug\ui\vimui.py", line 335, in destroy
    self.command('bwipeout ' + self.name)
  File "F:\Settings\home\.vim\vundle\vdebug\plugin/python\vdebug\ui\vimui.py", line 346, in command
    vim.command(cmd)
vim.error
```

This fixes these issues. I assume the problem was related to the fact that the last window in the tab was being closed.