TpBaseConnection: in CreateChannel, check that handle repo exists before using it
TpBaseConnection: in CreateChannel, check that handle repo exists before using it

This fixes a crash if a channel with an entirely unsupported handle type is
requested. The change is actually in utility code, so EnsureChannel will
also benefit from this once we implement it.

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