request_subscription: adjust variable naming for better assertion message
request_subscription: adjust variable naming for better assertion message

"iface != NULL" tells us very little, but "mutable_iface != NULL" is a
bit more useful.

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