qi.type.proxysignal: Fixes implicit access to a potentially destroyed object.
qi.type.proxysignal: Fixes implicit access to a potentially destroyed object.

In one of the lambdas in `ProxySignal` code was implicitly accessing the
`this` pointer instead of a dedicated `self` object copy of `this` and
was eventually causing an access to a potentially destroyed object.

Change-Id: I96fc0721758c75c2e49f11d361ab74591be8ed9f
Reviewed-on: http://gerrit2.aldebaran.lan/1319
Reviewed-by: jmonnon <8898eaf233516e520f996a244ca8a12d20f948b1@aldebaran.com>
Reviewed-by: gerrit <3b97070d46ccd66422a8e7b83e6967ed84780538@aldebaran-robotics.com>
Tested-by: gerrit <3b97070d46ccd66422a8e7b83e6967ed84780538@aldebaran-robotics.com>
Reviewed-by: philippe.martin <a3db8b74c3cf86a5cf091dca4e2125b498be933d@softbankrobotics.com>

aldebaran/libqi,aldebaran/libqi,aldebaran/libqi