aplay: change if statement to else if in xrun function

[GitHub pull_request - edited <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-utils pull request #50 was edited from JSFamily:

Honestly, It does not affect behavior in runtime. But I made this patch to help understanding the source code.

Request URL   : https://github.com/alsa-project/alsa-utils/pull/50
Patch URL     : https://github.com/alsa-project/alsa-utils/pull/50.patch
Repository URL: https://github.com/alsa-project/alsa-utils