AESinkALSA: Workaround alsa-lib buffer overflow in snd_pcm_chmap_print

Backport of PR #6065.

Provide a double-length buffer for snd_pcm_chmap_print(), which the
buggy versions cannot overflow.

We are not yet sure if this patch fixes ticket#15641, but this should go in regardless.

http://trac.kodi.tv/ticket/15641
http://mailman.alsa-project.org/pipermail/alsa-devel/2014-December/085815.html
(cherry picked from commit 957f8e2474ce733d9d05cb67b62721e9b3b1c389)