Fixed double free issue when destroying endpoint

Fixed double free issue when destroying endpoint

- after destroying memory, set NULL value to prevent double free
- add the usage of oc_mutex_lock() when block data is updated

Change-Id: I07dbdff8288888ece8f89f7b278e979c09cf8e51
Signed-off-by: hyuna0213.jo <hyuna0213.jo@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/15447
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Jaehong Jo <jaehong.jo@samsung.com>
Reviewed-by: jihwan seo <jihwan.seo@samsung.com>
Reviewed-by: Ashok Babu Channa <ashok.channa@samsung.com>