msm: msm_bus: Add NULL pointer checks.

Bus driver is crashing due to a NULL pointer
access sent by the client. A few NULL pointer
checks have been added to protect the driver
from invalid/NULL client handle.

CRs-Fixed: 458264
Change-Id: I822ad312ba1a0c0939d97eb92ea53094d408a836
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>