dnsdist: Actually try to read before checking if the socket is readable

dnsdist: Actually try to read before checking if the socket is readable

We need to because the TLS layer might already have data waiting
for us, while there might not be anything left on the OS-level
buffer associated to the socket.
If we don't ask the TLS layer, we might wait indefinitely for
something to arrive while the client has already sent everything,
and it's just waiting for us because the TLS record has been read.