(trunk, gtk) #5450: in transmission-gtk's main.c::on_app_exit(), check to see if shutdown's already been initiated so that we don't call tr_sessionClose() twice.

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@14154 f4695dd4-2c0a-0410-b89c-da849a56a58e