asoc: add null check for pcm pointer of snd_pcm_volume

In platform driver volume controls, add null check for
pcm pointer of struct snd_pcm_volume.

Change-Id: I511a79422eaeced6240849cbb665a289afdc5984
Signed-off-by: Xiaojun Sang <xsang@codeaurora.org>
2 files changed