Fix static analysis issue in keepalive

Fix static analysis issue in keepalive

- add return value checking logic after memory allocation

Change-Id: I1c28e13e01c2dd7bf4b78585adf0ffdc0e6ecdfd
Signed-off-by: Minji Park <minjii.park@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/14935
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>
Reviewed-by: Jaehong Jo <jaehong.jo@samsung.com>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-by: Ashok Babu Channa <ashok.channa@samsung.com>
(cherry picked from commit 64b44c48ab8944c7cae1cabf64aa0e1e98ada932)
Reviewed-on: https://gerrit.iotivity.org/gerrit/15339
Reviewed-by: jihwan seo <jihwan.seo@samsung.com>