[PATCH libxkbcommon] bench: Check for errors opening Compose file

[Ran Benita]

On Thu, Sep 15, 2016 at 02:12:38PM -0700, Bryce Harrington wrote:
> From: Bryce Harrington <bryce at bryceharrington.org>
> 
> Otherwise it can segfault e.g. running ./compose inside the bench
> directory.

Applied, thanks!

(I only notice now when replying that you used tabs instead of spaces,
but I'll fix it up myself).