NULL pointer check in cbor_test

NULL pointer check in cbor_test

cbor_test directly uses value returned by OCRepPayloadGetPropeString in memcmp .

The value is not checked before calling memcmp.

BUG: https://jira.iotivity.org/browse/IOT-1662
Change-Id: Ia998ab4154d0b2efb69dff232532d2b8b7f4f5de
Signed-off-by: Habib Virji <habib.virji@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/15365
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: George Nash <george.nash@intel.com>
Reviewed-by: Ziran Sun <ziran.sun@samsung.com>