IOT-2539 Fix return-type warning

IOT-2539 Fix return-type warning

The _worker function expects a void * return type
only functions with return type of void do not have
to have a return.

Since the return value does not appear to be used
simply return NULL.

Change-Id: I3cee6f7a491320d21b1d70f9a8df0f7b4538087b
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21737
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-by: Mats Wichmann <mats@linux.com>
Reviewed-by: Rick Bell <richard.s.bell@intel.com>
(cherry picked from commit 9a55ebdaefd544b37c7534b479ceff33ccc9f123)