qseecom: Check for case where listener service is not found

When listerner service is not in the list of registered listener,
the resulting ptr will a NULL.  Check for this case of NULL ptr
to indicate missing listener service.

Change-Id: I0d2c7b01143c40711f71bf434493ac5301314cf2
Signed-off-by: Mona Hossain <mhossain@codeaurora.org>