added the logic to clean receive buffer when disconnected

added the logic to clean receive buffer when disconnected

when connection state is disconnected,
received data has to be removed.

Change-Id: I05c5a0142549e3c3fac2fb83d6fd038487ec70c2
Signed-off-by: jihwan.seo <jihwan.seo@samsung.com>
Signed-off-by: Jaewook Jung <jw0213.jung@samsung.com>
Signed-off-by: Jaehong Jo <jaehong.jo@samsung.com>
Signed-off-by: jihwan.seo <jihwan.seo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/5209
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>