msm: synx: verify synx obj validity before accessing members

The change verifies that the synx object is valid before accessing
any of the members. This prevents clients access to synx objects
after they have been cleaned up.

Change-Id: I96e9e489af872fa1618fb66b942b263b7da1e52b
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>