audit: Checks valid value of audit_signals and tsk->audit_context

There can be a case when current task's audit context
may be NULL or even if it is not NULL, it can have
invalid non-zero value of dummy variable and also
we should audit the signal only if we have non-zero
value of audit_signals.

Issue has been seen during power on/off tests.
So, better to add explicit check on these variables.

Change-Id: I2f3e17d933fca8e3b8f4013f44f942f11c68f440
Signed-off-by: Chetan C R <cravin@codeaurora.org>