Fix GroupSync API of about GroupSync of Group feature.

Fix GroupSync API of about GroupSync of Group feature.

Crash is occured when som api of ThingsManager is called multiple times.

Fix [IOT-261], [IOT-264], [IOT-268]

Change-Id: Iec1cb11c570357411351c69450ace91ea6fa3754
Signed-off-by: HyunJun Kim <hyunjun2.kim@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/228
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>
(cherry picked from commit 135e8386212533f4c62e892f0695c78d945f2879)
Reviewed-on: https://gerrit.iotivity.org/gerrit/375
Reviewed-by: Sudarshan Prasad <sudarshan.prasad@intel.com>