lpm-levels: Add a NULL check of next event time

The next event value could be NULL if there is no event
queued. Add a NULL check to avoid wrong calculation of
next wakeup time.

Change-Id: I1ddd8db1803c8f88e99c050e8e40fb6cb5326bda
Signed-off-by: Venkat Devarasetty <vdevaras@codeaurora.org>