pcm: fix signed overflow in pcm_softvol

alsa-project/alsa-lib pull request #351 was opened from q66:

This fixes functionality on specific setups with UBSan.

Request URL : https://github.com/alsa-project/alsa-lib/pull/351 Patch URL : https://github.com/alsa-project/alsa-lib/pull/351.patch Repository URL: https://github.com/alsa-project/alsa-lib