TpChannel: do not disconnect from a TpProxySignalConnection that's already gone
TpChannel: do not disconnect from a TpProxySignalConnection that's already gone

This is incorrect, and in practice, causes a crash.

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib