ASoC: msm: handle null pointer access issue at audio client

During playback close, ioctl call to prepare for same
session is triggered resulting in access to null pointer
access. Add null pointer check before access audio
client and private data in prepare functions.

Change-Id: I88dee5526b4308a710535d966aa7aeceee668e18
Signed-off-by: Laxminath Kasam <lkasam@codeaurora.org>