Fix the logic in libpqrcv_receive() to determine if there's any incoming data

Fix the logic in libpqrcv_receive() to determine if there's any incoming data
that can be read without blocking. It used to conclude that there isn't, even
though there was data in the socket receive buffer. That lead walreceiver to
flush the WAL after every received chunk, potentially causing big performance
issues.

Backpatch to 9.0, because the performance impact can be very significant.