msm: msm_bus: Improve defensive checks for adhoc driver

Ensure that the current index is defaulted to 0 when unregistering
the client if the current index value is invalid. Also check
that bus find device returns a valid device pointer before proceeding.

Change-Id: I62cf5d0da9e2cc853b6e82cb9d029142d86c18e3
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>