Fixed memory leak from keep-alive

Fixed memory leak from keep-alive

The keep-alive mechanism was not properly deleting the payload
that was created for keep-alive.

Change-Id: I80736893e472c31f9058c2d2b5c1d3c38a692c04
Signed-off-by: hyuna0213.jo <hyuna0213.jo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/10223
Reviewed-by: jihwan seo <jihwan.seo@samsung.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jaehong Jo <jaehong.jo@samsung.com>
Reviewed-by: Ashok Babu Channa <ashok.channa@samsung.com>