Signal: fix race on signal destruction

Change-Id: I16db2dddd55610778ae1698ba82fd3bca5ef1d48
Reviewed-on: http://gerrit.aldebaran.lan/44783
Reviewed-by: tfontenay <tfontenay@aldebaran-robotics.com>
Reviewed-by: pdaouadi <pdaouadi@aldebaran-robotics.com>
Tested-by: pdaouadi <pdaouadi@aldebaran-robotics.com>