[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.