Fix crash when handling client service removal
Fix crash when handling client service removal

When processing a ClientServiceRemoved event, we usually only have a
USN, the Service instance is null. So we need to handle this correctly
and get the service type from the USN.

This fixes the following bug reported against Banshee:
https://bugzilla.gnome.org/show_bug.cgi?id=671708

mono/mono-upnp,mono/mono-upnp,wasabii/mono-upnp,wasabii/mono-upnp