wayland-server: Return 0 from read_events() in case of EAGAIN

Getting no data from the socket is not an error condition.  This may
happen in case of calling prepare_read() and then read_events() with
no other pending readers and no data in the socket.  In general,
read_events() may not queue up events in the given event queue.  From
a given threads point of view it doesn't matter whether events were
read and put in a different event queue or no events were read at all.