Fix minor issue for Group Action of ThingsManager.

Fix minor issue for Group Action of ThingsManager.

addActionSet and executeActionSet do not return error message when entered invalid parameters.
In this case, we return 'OC_STACK_INVALID_PARAM'.

getActionSetfromString didn't return when entered malformed string.
It will be returned NULL in this case.

Change-Id: Ifd28640d6aca6714646b33ea8fb69c55f8ca36f7
Signed-off-by: HyunJun Kim <hyunjun2.kim@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/631
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>