[Qt] Fix minor memory leak in slot execution

Reviewed by Allan Sandfeld Jensen.

Don't leak the "length" string when executing a slot in JavaScript.

* bridge/qt/qt_runtime.cpp:
(JSC::Bindings::QtConnectionObject::execute):

Change-Id: I7e7cd3c0bbe18e5345d9f024a95284c2c8ef09e9
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141420 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>