redis-cli pipe mode: don't stay in the write loop forever.

The code was broken and resulted in redis-cli --pipe to, most of the
times, writing everything received in the standard input to the Redis
connection socket without ever reading back the replies, until all the
content to write was written.

This means that Redis had to accumulate all the output in the output
buffers of the client, consuming a lot of memory.

Fixed thanks to the original report of anomalies in the behavior
provided by Twitter user @fsaintjacques.
