Fix a theoretical race listing pending messages

Fix a theoretical race listing pending messages

If you call ListPendingMessages before connecting to Received, the
following could theoretically occur:

    Empathy --- ListPendingMessages --> CM
    Empathy <-- LPM returns         --- CM
                                 <----- CM -----> Received
    Empathy connects to Received

and thus you could miss a message. Connecting to Received before calling
ListPendingMessages avoids this (mostly theoretical) bug.

Signed-off-by: Will Thompson <will.thompson@collabora.co.uk>