contact-manager: safely return if called after having cleaned up
contact-manager: safely return if called after having cleaned up

The OLPC activity manager's removed-service callback was looking up a
contact after the contact manager had cleaned up (but not disposed)
which was causing an assertion. The activity manager should arguably
disconnect from said signals...

Signed-off-by: Jonny Lamb <3d1fb3642aaab2ff70e3392a5ab916bf99eed6a3@collabora.co.uk>

freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut