pcm: fix stack allocation inside a loop

alsa-project/alsa-lib pull request #363 was opened from szsam:

Using alloca in a loop can lead to a stack overflow. Fix by using malloc and free.

Request URL : https://github.com/alsa-project/alsa-lib/pull/363 Patch URL : https://github.com/alsa-project/alsa-lib/pull/363.patch Repository URL: https://github.com/alsa-project/alsa-lib