slim-ngd: NULL pointer check for client buffer

There is the chance of passing NULL pointer by the client
during the Slimbus/Codec transactions. Hence add NULL
pointer check for client buffer aswell.

CRs-Fixed: 608324
Change-Id: I161e8f04401afebb64ed72d5f392a67feb6ec13a
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>