Dap: avoid NullReferenceException at shutdown if Mtp mounted (bgo#724645)
Dap: avoid NullReferenceException at shutdown if Mtp mounted (bgo#724645)

Give up unmapping device if DapService is already disposed...

arfbtwn/banshee,GNOME/banshee,GNOME/banshee,ixfalia/banshee,GNOME/banshee,GNOME/banshee,babycaseny/banshee,arfbtwn/banshee,babycaseny/banshee,arfbtwn/banshee,stsundermann/banshee,babycaseny/banshee,babycaseny/banshee,stsundermann/banshee,babycaseny/banshee,ixfalia/banshee,arfbtwn/banshee,Carbenium/banshee,stsundermann/banshee,Carbenium/banshee,babycaseny/banshee,ixfalia/banshee,arfbtwn/banshee,arfbtwn/banshee,ixfalia/banshee,GNOME/banshee,babycaseny/banshee,ixfalia/banshee,Carbenium/banshee,Carbenium/banshee,stsundermann/banshee,stsundermann/banshee,GNOME/banshee,ixfalia/banshee,ixfalia/banshee,GNOME/banshee,stsundermann/banshee,arfbtwn/banshee,ixfalia/banshee,arfbtwn/banshee,stsundermann/banshee,Carbenium/banshee,Carbenium/banshee,GNOME/banshee,babycaseny/banshee,stsundermann/banshee