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: I78d0d46e37333e099ec737ac4500a2f388d58c4e
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>