qcom: wdog: fix static analyzer reported issue in watchdog core

Code static analyzer reports 'pivot' variable is uninitialized.
Let's fix this by initializing it with NULL and add necessary
checks.

Change-Id: I2aa521002ea55fa1e44b3ce17262a9093dd130a0
Signed-off-by: Mukesh Ojha <mojha@codeaurora.org>