soc: qcom: subsystem_notif_virt: Add a NULL check and fix a data type

A NULL pointer check is added to prevent a NULL pointer
exception. A data type is fixed to prevent a type conversion
before an error check.

Change-Id: If4f6f70c8770ad48432e466e1b6a7e9bf4b62258
Signed-off-by: Anant Goel <anantg@codeaurora.org>