thermal: tsens: Check pointer validity before accessing its variables

Pointer is being dereferenced before its validity check. Make change to
have a check on the pointer validity before accessing its member
variables.

Change-Id: I271ff94bfe59430e69ea246f7e6ef90f7e0d6ac6
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>