pcm: Add error handler for `fgets`

[GitHub pull_request - opened <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-lib pull request #238 was opened from Korilakkuma:

`fgets` function returns `NULL` if error occured. Therefore, end loop in the case.

Request URL   : https://github.com/alsa-project/alsa-lib/pull/238
Patch URL     : https://github.com/alsa-project/alsa-lib/pull/238.patch
Repository URL: https://github.com/alsa-project/alsa-lib