dbus-1/: fix watching for multiple sources

When multiple sources are in use (ie: non-trivial profiles) then change
monitoring needs to watch multiple D-Bus object paths.  This involves
sending multiple AddMatch requests and receiving multiple replies.

The dbus-1 backend was only ever expecting to receive one reply, however
(apparently never having been used with multiple sources).  It freed its
internal state object after the first reply, causing memory access
errors on the subsequent replies.