qseecom: fix an issue when requested listener svc id does not exist

When listener svc id does not exist, the listener svc entry pointer
is actualy invalid, and should be set to NULL to avoid invalid pointer
deference on cache operation when sending listener resp to TZ.

Change-Id: I6af0c7405c9dc5dfe481396979023968a8324dd0
Signed-off-by: Zhen Kong <zkong@codeaurora.org>