[server] Compare ClientConnection instead of wl_client in SeatInterface

A downstream KWin test shows a possible heap-use-after-free if we
access the wl_client pointer. Basically we get the client disconnected
just before the focused surface gets unbind. Thus for a short moment
the ClientConnection pointer is gone. This needs to be extended with
a test case, but for the moment it should be good enough to get KWin
green again.