msm: synx: clear pointer on client destroy

The client reference stored in the user payload struct
can be accessed after free if the client closed its
session, as the pointer is not removed from payload
upon close.
Change ensures client structure is cleaned up on client
session closure to avoid any user-after-free.

Change-Id: I6df1604dd9f74f88f73bf3ef54f5a6befa4224c9
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>