Fix redmine #18485

Protect access to BoundObject::_currentSocket when used
from the SD's functions.

Change-Id: Ie079a527e70f24aacf4d6d345e127ead2bd36c7e
Reviewed-on: http://gerrit.aldebaran.lan/47874
Reviewed-by: tfontenay <tfontenay@aldebaran-robotics.com>
Reviewed-by: pdaouadi <pdaouadi@aldebaran-robotics.com>
Approved: tfontenay <tfontenay@aldebaran-robotics.com>
Tested-by: tfontenay <tfontenay@aldebaran-robotics.com>